ci(travis): prevent duplicate builds

The only branch that should be built is the master branch and PR's into
it. Prevent all other branches from triggering duplicate builds.

See https://github.com/travis-ci/travis-ci/issues/1147

.travis.yml

@@ -45,6 +45,10 @@ script:
   - yarn test-ci || travis_terminate 1
   - yarn coveralls || travis_terminate 1
 
+branches:
+  only:
+    - master
+
 jobs:
   include:
     - stage: deploy