Avoid double Travis builds

At the moment we build on both PRs and pushes which causes two builds
when you push to a branch with an open PR. In order to prevent this,
from now on it will only build for the master branch and PR branches.

travis-ci/travis-ci#1147

Author: jamesstonehill

.travis.yml

@@ -5,6 +5,9 @@ before_install:
   - gem update --system
   - gem install bundler
 cache: bundler
+branches:
+  only:
+    - master
 matrix:
   include:
   - rvm: 2.3.8