Allow guzzle6 v2 adapter (#556)

Author: barryvdh

composer.json

@@ -23,7 +23,7 @@
     "require": {
         "php": "^5.6|^7",
         "omnipay/common": "^3",
-        "php-http/guzzle6-adapter": "^1.1"
+        "php-http/guzzle6-adapter": "^1.1|^2"
     },
     "require-dev": {
         "omnipay/tests": "^3"