title: How to configure Travis-CI to build pull requests & merges to master w/o redundancy
body: <p>To put it in "BDD" terms:</p>
  
  <blockquote>
    <p>Background: <br/>
    Given I'm contributing to a GH repo<br/></p>
    
    <p>When I create a pull request<br/>
    Then Travis should build the latest commit</p>
    
    <p>When I push to an existing pull request<br/>
    Then Travis should build the latest commit</p>
    
    <p>When I merge a pull request to master <br/>
    Then Travis should build master</p>
  </blockquote>
  
  <p>I was confused by Travis-CI's "build pushes" and "build PRs" settings, as:</p>
  
  <ul>
  <li>Enabling both causes each Pull Request to be build twice by Travis
  
  <ul>
  <li>once for the commit on that branch</li>
  <li>and once again for the merge commit of that branch into its destination</li>
  </ul></li>
  <li>Enabling just "build PRs" causes PRs to be built, but doesn't result in post-merge builds (i.e. on master).</li>
  <li>Enabling "pushes" brute-force satisfies the above criteria by building <em>all</em> pushes to the repo. You can try to finagle things by white- &amp; black-listing branches, but that will probably bite you unless you're rigorously disciplined with branch names.</li>
  </ul>
  
  <p>This is explained more in <a href="http://docs.travis-ci.com/user/pull-requests/">Travis-CI docs</a> and <a href="https://github.com/travis-ci/travis-ci/issues/3241">GH issue #3241</a>.</p>
  
  <p>Anyone know a configuration that satisfies the above criteria?</p>