Upgrading dependencies to check backwards compatibility in automated tests

.travis.yml

@@ -8,6 +8,7 @@ php:
   - 5.3
   - 5.4
   - 5.5
+  - 5.6
 
 before_script:
   - composer self-update

composer.json

@@ -14,10 +14,10 @@
     "minimum-stability": "stable",
     "require": {
         "php": ">=5.3.0",
-        "symfony/config": "~2.3",
-        "symfony/dependency-injection": "~2.3",
-        "symfony/filesystem": "~2.3",
-        "symfony/yaml": "~2.3",
+        "symfony/config": "~2.3|~3.0",
+        "symfony/dependency-injection": "~2.3|~3.0",
+        "symfony/filesystem": "~2.3|~3.0",
+        "symfony/yaml": "~2.3|~3.0",
         "twig/twig": "~1.16",
         "jakoch/phantomjs-installer": "~2.1"
     },