title: Install gem for all installed ruby versions?
body: <p>I have a setup script that downloads the files for a number of rails apps.  For various reasons, the apps are on different versions and it needs to remain that way. </p>
  
  <p>It is possible to search the .ruby-versions files to determine all the required ruby versions and then iterate through them and run rbenv install  on each. Is there a quick way to run <code>gem install bundler</code> for every ruby version without changing the local version and doing it manually?</p>