Also run tests against PHP 5.6 and PHP 7

Author: JeroenDeDauw

.travis.yml

@@ -3,6 +3,8 @@ language: php
 php:
   - 5.4
   - 5.5
+  - 5.6
+  - 7
 
 before_script:
-  - composer install
+  - composer install