Merge pull request #62 from edx/benp/push-config-master-only

Do not build other branches unless they are part of a PR.

.travis.yml

@@ -6,6 +6,9 @@ script:
     - make validate
     - git fetch origin master:refs/remotes/origin/master # https://github.com/edx/diff-cover#troubleshooting
     - make diff.report
+branches:
+    only:
+      - master
 after_success:
     - coveralls
     - bash ./scripts/build-stats-to-datadog.sh