Update travis file

.travis.yml

@@ -5,7 +5,14 @@ php:
   - 5.4
   - 5.5
 
+env:
+  - SYMFONY_VERSION=2.2.*
+  - SYMFONY_VERSION=2.3.*
+  - SYMFONY_VERSION=2.4.*
+  - SYMFONY_VERSION=dev-master
+
 before_script:
+  - composer require symfony/symfony:${SYMFONY_VERSION} --prefer-source
   - composer install --dev --prefer-source
 
 script: phpunit