Add a dev requirement to force Silex to use HttpKernel 2.4+

Using HttpKernel 2.3 with HttpFoundation 2.4 makes Silex create a RequestStack
but which is not updated by the kernel, which breaks our lowest-bound test.

composer.json

@@ -25,6 +25,7 @@
     "require-dev":  {
         "pimple/pimple": "~1.0",
         "symfony/http-foundation": "~2.4|~3.0",
+        "symfony/http-kernel": "~2.4|~3.0",
         "symfony/phpunit-bridge": "~3.3",
         "symfony/routing": "~2.3|~3.0",
         "silex/silex":     "~1.0",