Allow phpunit 5.7

Author: barryvdh

composer.json

@@ -29,7 +29,7 @@
         "php-http/mock-client": "^1",
         "php-http/guzzle6-adapter": "^1.1.1",
         "mockery/mockery": "^1",
-        "phpunit/phpunit": "^6"
+        "phpunit/phpunit": "^5.7|^6"
     },
     "require-dev": {
         "omnipay/common": "^3",
@@ -40,6 +40,5 @@
             "dev-master": "3.0.x-dev"
         }
     },
-    "minimum-stability": "RC",
     "prefer-stable": true
 }