Only run test when committing to a master branch.

This avoids running them twice for each PR update.

Author: helmo

.travis.yml

@@ -2,6 +2,11 @@ language: generic
 
 sudo: required
 
+# Only run test when committing to a master branch.
+branches:
+  only:
+    - 7.x-3.x
+
 env:
   global:
     - DOCKER_VERSION=1.11.2-0~trusty