title: Bundling different versions of a development gem depending on the ruby version
body: <p>I have an application that depends on <code>flexmock</code>. For a number of reasons, I want this app to run on 1.8.7 to 2.0.0 and beyond, so far it worked really well. Since the app has a huge test suite it's been easy to maintain compatibility.</p>
  
  <p>The only issue is <code>flexmock</code>. There is a bug in the older version of flexmock that I have to work around with a monkeypatch. The newer version of flexmock fixes the bug, but it only works on 1.9.3. The older version of flexmock that I use also gives test failures on 2.0.0 since it incorrectly records the number of method calls on classes like Time.</p>
  
  <p>Is there a magic Bundler incantation that I can use to modify my development dependencies so that they come out with relevant versions for each version of Ruby I run on? Specifically so that it runs clean on Travis-Ci?</p>
  
  <p>I do not version-control <code>Gemfile.lock</code></p>