title: Specify git path in gemspec add_dependency entry
body: <p>I have git-forked two gems (say <em>foo</em> and <em>bar</em>), one depending on the other:</p>
  
  <p><strong>foo.gemspec</strong></p>
  
  <pre><code>...
  s.add_dependency "bar"
  </code></pre>
  
  <p>I want my fork of <em>foo</em> to depend on my fork of <em>bar</em>, not on the original <em>bar</em> gem.</p>
  
  <p>However this doesn't work:</p>
  
  <pre><code>s.add_dependency "bar", git: 'git:github.com/vemv/bar.git' # =&gt; ArgumentError!
  </code></pre>
  
  <p><strong>How to specify the source of a dependency in a .gemspec file?</strong></p>
  
  <p><strong>If not possible, what to do instead?</strong></p>