title: Gem install error: 'Please specify at least one gem name'
body: <p>When I run 'gem install bundler' for example, it works fine if I am running ruby version 1.9.3, but when running ruby 2.1.2, it seems not to see the 'bundler' arg and gives me</p>
  
  <pre><code>ERROR:  While executing gem ... (Gem::CommandLineError)
    Please specify at least one gem name (e.g. gem build GEMNAME)
  </code></pre>
  
  <p>I see this on two different systems running rbenv.</p>
  
  <p>Here is the last part of the output when running with RBENV_DEBUG=1:</p>
  
  <pre><code>+ [rbenv-exec:45] export PATH=/home/ded/.rbenv/versions/2.1.2/bin:/home/ded/.rbenv  /libexec:/home/ded/.rbenv/plugins/bundler/bin:/home/ded/.rbenv/plugins/ruby-build/bin:.bundle/bin:/home/ded/.rbenv/shims:/home/ded/.rbenv/bin:/home/ded/.cabal/bin:/home/ded/.emacs/ded/lisp/ecukes:/home/ded/local/node/bin:/home/ded/j64-701/bin:/home/ded/bin:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/local/plan9/bin
  + [rbenv-exec:45] PATH=/home/ded/.rbenv/versions/2.1.2/bin:/home/ded/.rbenv/libexec:/home/ded/.rbenv/plugins/bundler/bin:/home/ded/.rbenv/plugins/ruby-build/bin:.bundle/bin:/home/ded/.rbenv/shims:/home/ded/.rbenv/bin:/home/ded/.cabal/bin:/home/ded/.emacs/ded/lisp/ecukes:/home/ded/local/node/bin:/home/ded/j64-701/bin:/home/ded/bin:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/local/plan9/bin
  + [rbenv-exec:47] exec -a gem /home/ded/.rbenv/versions/2.1.2/bin/gem install bundler
    ERROR:  While executing gem ... (Gem::CommandLineError)
      Please specify at least one gem name (e.g. gem build GEMNAME)
  </code></pre>
  
  <p>Any suggestions what could be going wrong?</p>