Only build pushes to master and stable branches

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

.travis.yml

@@ -23,6 +23,12 @@ env:
   matrix:
   - DB=sqlite
 
+branches:
+  only:
+  - master
+  - "/^stable\\d+(\\.\\d+)?$/"
+  - /^v\d++(\.\d+)?+(\.\d+)?+(\.\d+)?$/
+
 cache:
   directories:
   - "$HOME/.composer/cache/files"