title: Podfile - specify sub dependency version
body: <p>Is there a way to force a sub dependency version for a Pod dependency? I have this Pod dependency;</p>
  
  <pre><code>Podfile
  ...
  pod 'Folly', :podspec =&gt; '../node_modules/react-native/third-party-podspecs/Folly.podspec'
  </code></pre>
  
  <p>It has a subdependncy on boost-for-react-native without specifying a version which results in 1.57.0 being installed.</p>
  
  <pre><code>Podfile.lock
  ...
    - Folly (2016.09.26.00):
      - boost-for-react-native
  </code></pre>
  
  <p>I need version 1.63.0. If I specify that in the parent Podfile I still end up with 1.57.0. </p>
  
  <pre><code>Podfile
  ...
  </code></pre>
  
  <p>pod 'boost-for-react-native', '~> 1.63.0' </p>
  
  <p>I believe I need to specify it on the Folly file itself.</p>