Updated dependencies to only be available in dev

Author: RonRademaker

composer.json

@@ -4,7 +4,9 @@
     "type": "library",
     "require": {
         "swiftmailer/swiftmailer": "^5.4",
-        "twig/twig": "^1.23",
+        "twig/twig": "^1.23"
+    },
+    "require-dev": {
         "phpunit/phpunit": "^4.7",
         "Mockery/Mockery": "^0.9.4"
     },