only build pushes to the master branch

Branches in pull requests will still be built by virtue of being
associated with a pull request. Restricting "branches" to "only master"
will prevent duplicate web hook firing to Travis.

Signed-off-by: Robb Kidd <robb@thekidds.org>

Author: robbkidd

.travis.yml

@@ -1,4 +1,7 @@
 sudo: false
+branches:
+  only:
+    - master
 language: ruby
 bundler_args: "--jobs 7"