Issue #10 support symfony/property-access v3 or v4

Author: judgej

composer.json

@@ -26,7 +26,7 @@
     "require": {
         "omnipay/common": "^3",
         "academe/authorizenet-objects": "^0.7",
-        "symfony/property-access": "^3.2"
+        "symfony/property-access": "^3.2 || ^4.0"
     },
     "require-dev": {
         "omnipay/tests": "^3",