title: how to install specific branch of git repository using yarn?
body: <p>I am able to install one GitHub package but not other using yarn. Please let me know what could be the issue here.</p>
  <p>I can add <a href="https://github.com/fancyapps/fancybox#3.0" rel="noreferrer">https://github.com/fancyapps/fancybox#3.0</a>
  but not <a href="https://github.com/opentripplanner/otp-react-redux#result-post-processor" rel="noreferrer">https://github.com/opentripplanner/otp-react-redux#result-post-processor</a></p>
  <pre><code>ravis-MacBook-Pro:gitprojects ******$ mkdir test
  ravis-MacBook-Pro:gitprojects ******$ cd test
  ravis-MacBook-Pro:test ***********$ yarn init
  yarn init v1.6.0
  question name (test): 
  question version (1.0.0): 
  question description: 
  question entry point (index.js): 
  question repository url: 
  question author: 
  question license (MIT): 
  question private: 
  success Saved package.json
  ✨  Done in 11.54s.
  
  ravis-MacBook-Pro:test ******$ yarn add https://github.com/fancyapps/fancybox#3.0
  yarn add v1.6.0
  info No lockfile found.
  [1/4]   Resolving packages...
  [2/4]   Fetching packages...
  [3/4]   Linking dependencies...
  warning &quot; &gt; @fancyapps/fancybox@3.0.48&quot; has unmet peer dependency &quot;jquery@&gt;=1.9.0&quot;.
  [4/4]   Building fresh packages...
  success Saved lockfile.
  success Saved 1 new dependency.
  info Direct dependencies
  └─ @fancyapps/fancybox@3.0.48
  info All dependencies
  └─ @fancyapps/fancybox@3.0.48
  ✨  Done in 1.35s.
  
  
  ravis-MacBook-Pro:test *******$ yarn add https://github.com/opentripplanner/otp-react-redux#result-post-processor
  yarn add v1.6.0
  [1/4]   Resolving packages...
  error Can't add &quot;otp-react-redux&quot;: invalid package version undefined.
  info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.
  ravis-MacBook-Pro:test *******$ 
  </code></pre>