title: rbenv not able to install ruby 2.2.5 and 2.3.1
body: <p>I run <code>brew update</code> and <code>brew upgrade rbenv ruby-build</code>. FYI: I updated xcode to version 8.1 yesterday.</p>
  
  <p>I checked available versions, <code>rbenv install -l</code>.</p>
  
  <p>Then I tried to install ruby2.2.5 by <code>rbenv install 2.2.5</code>, then I get the following errors.</p>
  
  <p>I tried 2.3.1 but I got the same errors. How can I fix the problem?</p>
  
  <pre><code>Downloading ruby-2.2.5.tar.bz2...
  -&gt; https://cache.ruby-lang.org/pub/ruby/2.2/ruby-2.2.5.tar.bz2
  Installing ruby-2.2.5...
  
  BUILD FAILED (OS X 10.11.6 using ruby-build 20160602)
  
  Inspect or clean up the working tree at /var/folders/2p/49l6q0gs6qzgtxbg4h1st_5h0000gq/T/ruby-build.20161106122840.51330
  Results logged to /var/folders/2p/49l6q0gs6qzgtxbg4h1st_5h0000gq/T/ruby-build.20161106122840.51330.log
  
  Last 10 log lines:
    Referenced from: /private/var/folders/2p/49l6q0gs6qzgtxbg4h1st_5h0000gq/T/ruby-build.20161106122840.51330/ruby-2.2.5/./miniruby (which was built for Mac OS X 10.12)
    Expected in: /usr/lib/libSystem.B.dylib
  
  dyld: Symbol not found: _clock_gettime
    Referenced from: /private/var/folders/2p/49l6q0gs6qzgtxbg4h1st_5h0000gq/T/ruby-build.20161106122840.51330/ruby-2.2.5/./miniruby (which was built for Mac OS X 10.12)
    Expected in: /usr/lib/libSystem.B.dylib
  
  make: *** [encdb.h] Trace/BPT trap: 5
  make: *** Waiting for unfinished jobs....
  make: *** [.rbconfig.time] Trace/BPT trap: 5
  </code></pre>