Configure Travis build matrix

Author: Benoît Merlet

.travis.yml

@@ -2,14 +2,22 @@ language: php
 
 php:
   - 5.3
+  - 5.4
   - 5.5
+  - 5.6
+
+env:
+  - SYMFONY_VERSION="2.3.*"
+  - SYMFONY_VERSION="2.4.*"
+  - SYMFONY_VERSION="2.5.*"
+  - SYMFONY_VERSION="2.6.*"
 
 before_script:
-  - composer install
+  - composer require symfony/http-kernel:${SYMFONY_VERSION}
 
 script:
   - phpunit
 
 branches:
   only:
-      - master
+    - master