Merge pull request #17 from edx/benp/only-master

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

.travis.yml

@@ -4,6 +4,9 @@ install:
     - "pip install coveralls"
     - "pip install -r requirements.txt"
 script: make test quality
+branches:
+    only:
+      - master
 after_success:
     - coveralls
     - bash ./scripts/build-stats-to-datadog.sh