title: install_bitbucket or install_github and install specific versions of dependencies
body: <p>I have a package hosted on a bitbucket private repository and would like to install my package along with the specific versions of it's dependencies specified in the Depends section of the DESCRIPTION file:</p>
  
  <pre><code>Depends: R (&gt;= 2.15.0), foo (== 1.17-12) 
  </code></pre>
  
  <p>Is there a way to have devtools automatically find and install foo version 1.17-12 for R 2.15.0?</p>
  
  <p>I like devtools' ability to install different refs (development, master, my-feature-branch, ...etc.) from my bitbucket repo and then install all of it's dependencies, but installing my R package on different servers at different times, means I can easily pick up unwanted dependent package changes and I also need to keep R up to date all of the time in order for it to find all of the packages.</p>