Bump minimum Symfony version to 2.7

Since #58 I discovered that we do not really support Symfony <2.7, since TokenStorageInterface and AuthorizationCheckerInterface were implemented later than 2.4, in 2.6.
Since 2.6 and previous versions are now no longer supported, it makes sense to bump to the oldest currently supported version.

Author: Jean85

composer.json

@@ -14,12 +14,12 @@
     "require": {
         "php": ">=5.3.3",
         "sentry/sentry": ">=1.5.0",
-        "symfony/config": "^2.4|^3.0",
-        "symfony/console": "^2.4|^3.0",
-        "symfony/dependency-injection": "^2.4|^3.0",
-        "symfony/event-dispatcher": "^2.4|^3.0",
-        "symfony/http-kernel": "^2.4|^3.0",
-        "symfony/security-core": "^2.4|^3.0"
+        "symfony/config": "^2.7|^3.0",
+        "symfony/console": "^2.7|^3.0",
+        "symfony/dependency-injection": "^2.7|^3.0",
+        "symfony/event-dispatcher": "^2.7|^3.0",
+        "symfony/http-kernel": "^2.7|^3.0",
+        "symfony/security-core": "^2.7|^3.0"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^1.8.0",