be explicit about required dependencies

Depending on the FrameworkBundle implicitly pulling into all the
required packages is fragile and likely to break.

Author: xabbuh

composer.json

@@ -14,7 +14,12 @@
     "require": {
         "php": ">=5.3.3",
         "sentry/sentry": ">=1.5.0",
-        "symfony/framework-bundle": ">=2.4.0"
+        "symfony/config": "^2.4|^3.0|^4.0",
+        "symfony/console": "^2.4|^3.0|^4.0",
+        "symfony/dependency-injection": "^2.4|^3.0|^4.0",
+        "symfony/event-dispatcher": "^2.4|^3.0|^4.0",
+        "symfony/http-kernel": "^2.4|^3.0|^4.0",
+        "symfony/security-core": "^2.4|^3.0|^4.0"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^1.8.0",