title: Bundler error message "Revision master does not exist in the repository"
body: <p>When trying to install the gem "u2f" from the repo at "castle/ruby-u2f" I get an error message:</p>
  
  <pre><code>Fetching git://github.com/castle/ruby-u2f.git
  fatal: Needed a single revision
  Revision master does not exist in the repository git://github.com/castle/ruby-u2f.git. Maybe you misspelled it?
  </code></pre>
  
  <p>I cannot find any information about this error message. The text does not appear in any of the bundler source files, so I don't even know where to start debugging.</p>
  
  <p>If anyone has any insight into the source and cause of this error, I'd love to know.</p>