Update dependencies

Author: amacneil

composer.json

@@ -26,20 +26,14 @@
         "psr-0": { "Omnipay\\Stripe\\" : "src/" }
     },
     "require": {
-        "omnipay/common": "2.*"
+        "omnipay/common": "~2.0"
     },
     "require-dev": {
-        "guzzle/plugin-mock": "~3.1",
-        "mockery/mockery": "~0.8",
-        "omnipay/tests": "2.*",
-        "phpunit/phpunit": "~3.7.16",
-        "squizlabs/php_codesniffer": "~1.4.4"
+        "omnipay/tests": "~2.0"
     },
     "extra": {
         "branch-alias": {
             "dev-master": "2.0.x-dev"
         }
-    },
-    "minimum-stability": "dev",
-    "prefer-stable": true
+    }
 }