Change abandoned guzzle/http to guzzle/guzzle (fixes #33)

Composer shows warnings in any project requiring omnipay-common, since guzzle/http is abandoned and no longer supported. This is not nice, esp. since this might introduce some security issues in future.

composer.json

@@ -28,7 +28,7 @@
     },
     "require": {
         "php": ">=5.3.2",
-        "guzzle/http": "~3.1",
+        "guzzle/guzzle": "~3.9",
         "symfony/http-foundation": "~2.1"
     },
     "require-dev": {