Update .travis.yml

Let Travis build branches master and gh-pages only

.travis.yml

@@ -8,6 +8,11 @@ php:
   - hhvm
   - nightly
 
+branches:
+  only:
+    - master
+    - gh-pages
+
 script:
   - find . -type f -name '*.php' -print0 | xargs -0 -I file php -l file > /dev/null