title: opam upgrade wants to downgrade a bunch of packages
body: <p>I just tried an 'opam upgrade' and got (the final summary):</p>
  
  <pre><code>1 to install | 59 to reinstall | 3 to upgrade | 34 to downgrade | 0 to remove
  Do you want to continue ? [Y/n]
  </code></pre>
  
  <p>Downgrading 34 packages makes me nervous. Why would it want to do this?</p>
  
  <p>Some examples of packages it wanted to downgrade:</p>
  
  <ul>
  <li>downgrade llvm.3.2 to 3.1</li>
  <li>downgrade ocamlfind.1.4.0 to 1.3.3 [required by bitstring, camltc, ezjsonm, fat-filesystem, google-drive-ocamlfuse, merlin, mirage-block-xen, mirage-console-xen, mirari, utop]</li>
  <li>downgrade SPOC.130624 to 121217</li>
  <li>downgrade bitstring.2.0.4 to 2.0.3</li>
  <li><p>downgrade camlzip.1.05 to 1.04 [required by google-drive-ocamlfuse]</p></li>
  <li><p>downgrade ezjsonm.0.2.0 to 0.1.0</p></li>
  <li>downgrade gapi-ocaml.0.2.1 to 0.2 [required by google-drive-ocamlfuse]</li>
  <li>downgrade merlin.1.5 to 1.3</li>
  <li>downgrade utop.1.10 to 1.9</li>
  </ul>
  
  <p>Of course, this makes me kind of nervous, so I answered 'n'. Anyway to fix this?</p>
  
  <p>opam version is 1.1.0.</p>
  
  <p>EDIT: I figured I may as well post the whole result:</p>
  
  <pre><code>$ opam upgrade
  *The brute-force exploration algorithm timed-out [108 states, 5s].
  You might need to add explicit version constraints to your request to get a better answer.*
  
  The following actions will be performed:
   - install obuild.0.0.2 [required by mirari]
   - downgrade llvm.3.2 to 3.1
   - downgrade ocamlfind.1.4.0 to 1.3.3 [required by bitstring, camltc, ezjsonm, fat-filesystem,     google-drive-ocamlfuse, merlin, mirage-block-xen, mirage-console-xen, mirari, utop]
   - downgrade SPOC.130624 to 121217
   - downgrade bitstring.2.0.4 to 2.0.3
   - downgrade camlzip.1.05 to 1.04 [required by google-drive-ocamlfuse]
   - recompile camomile.0.8.5 [use ocamlfind]
   - recompile cmdliner.0.9.2 [use ocamlfind]
   - downgrade cryptokit.1.9 to 1.6 [required by google-drive-ocamlfuse]
   - recompile dypgen.20120619-1 [use ocamlfind]
   - recompile easy-format.1.0.1 [use ocamlfind]
   - recompile extlib-compat.1.6.1 [use ocamlfind]
   - downgrade fd-send-recv.1.0.1 to 1.0.0 [required by mirari]
   - downgrade herelib.109.35.02 to 109.35.00 [required by fat-filesystem, mirage-block-xen, mirage-console-xen]
   - recompile ipaddr.1.0.0 [use ocamlfind]
   - recompile lablgtk.2.16.0 [use ocamlfind]
   - recompile menhir.20130912 [use ocamlfind]
   - recompile mirage-types.1.0.0 [use ocamlfind]
   - recompile ocamlfuse.2.7.1-cvs [use ocamlfind]
   - downgrade ocurl.0.6.0 to 0.5.5 [required by google-drive-ocamlfuse]
   - recompile omake.0.9.8.6-0.rc1 [use ocamlfind]
   - recompile omd.0.9.0 [use ocamlfind]
   - recompile optcomp.1.5 [use ocamlfind]
   - downgrade ounit.2.0.0 to 1.1.2 [required by camltc, fat-filesystem, mirage-block-xen, mirage-console-xen]
   - recompile parmap.1.0-rc3 [use ocamlfind]
   - recompile pipebang.109.28.02 [use ocamlfind]
   - recompile re.1.2.1 [use ocamlfind]
   - recompile react.0.9.4 [use ocamlfind]
   - downgrade res.4.0.4 to 3.2.0 [required by fat-filesystem, mirage-block-xen, mirage-console-xen]
   - recompile sqlite3-ocaml.2.0.5 [use ocamlfind]
   - recompile ssl.0.4.6 [use ocamlfind]
   - downgrade type_conv.109.53.02 to 109.53.00 [required by fat-filesystem, mirage-block-xen, mirage-console-xen]
   - recompile ulex.1.1 [use ocamlfind]
   - recompile uutf.0.9.3 [use ocamlfind]
   - downgrade xmlm.1.2.0 to 1.1.1 [required by google-drive-ocamlfuse]
   - downgrade biniou.1.0.6 to 1.0.5 [required by google-drive-ocamlfuse, merlin]
   - recompile tuntap.0.7.0 [use ocamlfind]
   - recompile ocamldot.1.0 [use ocamlfind]
   - recompile ocamlgraph.1.8.3 [use ocamlfind]
   - recompile mirage-clock-unix.1.0.0 [use ocamlfind]
   - recompile mirage-clock-xen.1.0.0 [use ocamlfind]
   - downgrade camlimages.4.1.1 to 4.0.1
   - downgrade ocplib-endian.0.4 to 0.3 [required by fat-filesystem, mirage-block-xen, mirage-console-xen]
  - downgrade uri.1.3.11 to 1.3.10
  - recompile zed.1.2 [use ocamlfind]
  - downgrade lwt.2.4.4 to 2.4.3 [required by camltc, fat-filesystem, mirage-block-xen, mirage-console-xen, utop]
  - recompile ocamlnet.3.7.3 [use ocamlfind]
  - recompile bin_prot.109.53.02 [use ocamlfind]
  - recompile comparelib.109.27.02 [use ocamlfind]
  - recompile dyntype.0.9.0 [use ocamlfind]
  - downgrade fieldslib.109.20.02 to 109.20.00 [required by fat-filesystem, mirage-block-xen, mirage-console-xen]
   - recompile pa_ounit.109.53.02 [use ocamlfind]
   - downgrade sexplib.109.55.02 to 109.55.00 [required by fat-filesystem, mirage-block-xen, mirage-console-xen]
   - downgrade variantslib.109.15.02 to 109.15.00 [required by fat-filesystem, mirage-block-xen, mirage-console-xen]
   - recompile jsonm.0.9.1 [use ocamlfind]
  - downgrade yojson.1.1.6 to 1.0.3 [required by google-drive-ocamlfuse, merlin]
   - downgrade mirari.0.9.7 to 0.9.3
   - recompile patoline.0.1 [use ocamlfind]
   - upgrade camltc.0.8.1 to 0.8.2
   - downgrade lambda-term.1.5 to 1.4 [required by utop]
   - recompile cow.0.9.1 [use ocamlfind]
   - recompile pa_bench.109.55.02 [use ocamlfind]
   - recompile custom_printf.109.27.02 [use ocamlfind]
   - recompile typerep.109.55.02 [use ocamlfind]
   - downgrade ezjsonm.0.2.0 to 0.1.0
   - downgrade gapi-ocaml.0.2.1 to 0.2 [required by google-drive-ocamlfuse]
   - downgrade merlin.1.5 to 1.3
   - downgrade utop.1.10 to 1.9
  - recompile core_kernel.109.55.02 [use ocamlfind]
   - downgrade google-drive-ocamlfuse.0.5.2 to 0.4.4
   - recompile pa_test.109.53.02 [use ocamlfind]
   - recompile core.109.55.02 [use ocamlfind]
  - recompile async_core.109.55.02 [use ocamlfind]
  - recompile async_unix.109.55.02 [use ocamlfind]
   - recompile async_extra.109.55.02 [use ocamlfind]
  - recompile async.109.53.02 [use ocamlfind]
   - recompile cstruct.1.0.1 [use ocamlfind]
   - recompile gen_server.2.0.1 [use ocamlfind]
   - upgrade cohttp.0.9.14 to 0.9.16
   - recompile io-page-unix.0.9.9 [use ocamlfind]
   - recompile io-page-xen.0.9.9 [use ocamlfind]
   - recompile mirage-fs-unix.1.0.0 [use ocamlfind]
   - downgrade shared-memory-ring.1.0.0 to 0.4.3 [required by mirage-block-xen, mirage-console-xen]
   - recompile xenstore.1.2.5 [use ocamlfind]
   - recompile cowabloga.0.0.3 [use ocamlfind]
   - recompile mirage-http-unix.1.0.0 [use ocamlfind]
   - downgrade mirage-block-unix.1.2.0 to 1.1.0 [required by fat-filesystem]
   - recompile mirage-net-unix.1.0.0 [use ocamlfind]
   - upgrade mirage.1.0.3 to 1.0.4
   - recompile mirage-unix.1.0.0 [use ocamlfind]
   - recompile mirage-xen.1.0.0 [use ocamlfind]
   - downgrade fat-filesystem.0.10.0 to 0.6.2
   - recompile mirage-console-unix.1.0.0 [use ocamlfind]
   - recompile mirage-tcpip-unix.0.9.5 [use ocamlfind]
   - downgrade mirage-block-xen.1.0.0 to 0.4.0
   - downgrade mirage-console-xen.1.0.1 to 1.0.0
    - recompile mirage-www.1.0.0 [use ocamlfind]
  1 to install | 59 to reinstall | 3 to upgrade | 34 to downgrade | 0 to remove
  Do you want to continue ? [Y/n] 
  </code></pre>