Update to Symfony 3

Author: Jonas De Keukelaere

composer.json

@@ -14,9 +14,9 @@
   "require-dev": {
     "php": "^5.5 || ^7.0",
 
-    "symfony/symfony": "^2.8",
-    "symfony/framework-bundle": "^2.8",
-    "symfony/monolog-bundle": "^2.8",
+    "symfony/symfony": "^3.3",
+    "symfony/framework-bundle": "^3.0",
+    "symfony/monolog-bundle": "^3.1",
     "sensio/framework-extra-bundle": "~3.0",
     "symfony/phpunit-bridge": "^3.0",
     "phpunit/phpunit": "^5.2 || ^4.8"