Added Symfony 4 support

Author: yohang

composer.json

@@ -17,17 +17,17 @@
         }
     ],
     "require": {
-        "php":                      ">=5.3.9",
-        "symfony/options-resolver": "^2.6|^3.0",
-        "symfony/event-dispatcher": "^2.6|^3.0",
-        "symfony/property-access":  "^2.6|^3.0"
+        "php":                      ">=5.5",
+        "symfony/options-resolver": "^2.6|^3.0|^4.0",
+        "symfony/event-dispatcher": "^2.6|^3.0|^4.0",
+        "symfony/property-access":  "^2.6|^3.0|^4.0"
     },
     "require-dev": {
         "phpunit/phpunit":              "^4.0",
         "pimple/pimple":                "^1.0",
-        "symfony/dependency-injection": "^2.6|^3.0",
-        "symfony/framework-bundle":     "^2.6|^3.0",
-        "symfony/security":             "^2.6|^3.0",
+        "symfony/dependency-injection": "^2.6|^3.0|^4.0",
+        "symfony/framework-bundle":     "^2.6|^3.0|^4.0",
+        "symfony/security":             "^2.6|^3.0|^4.0",
         "twig/twig":                    "^1.13"
     },
     "suggest": {