Run travis build also on PHP 7 builds

See https://github.com/travis-ci/travis-ci/issues/2480#issuecomment-75988205

.travis.yml

@@ -6,6 +6,7 @@ php:
   - 5.4
   - 5.5
   - 5.6
+  - 7
   - hhvm
 
 env: TMPDIR=/tmp