updated Http-Kernel dependency

Closes silexphp/Silex-WebProfiler#94.

Author: HeahDude

composer.json

@@ -17,7 +17,7 @@
     ],
     "require": {
         "php": ">=5.5.9",
-        "symfony/http-kernel": "~2.8|~3.0",
+        "symfony/http-kernel": "~3.1",
         "symfony/routing": "~2.8|~3.0",
         "symfony/twig-bridge": "~2.8|~3.0"
     },