Removing libxml dependency

libxml-ruby was added in this commit by Myles:

https://github.com/pinpayments/spreedly-core-ruby/commit/cae5a33af3f787312c6d5dfee7e3a03362041303

And has since been removed by this commit by Justin:

https://github.com/pinpayments/spreedly-core-ruby/commit/18e7f4585399f4b012e73bd036849d6a64c21f5d

The message_key stuff was removed, but the dependency was not. I found this issue by trying to setup `gateway` on my iMac at home, and it just _doesnt_ want to install libxml for some reason. After this change, I'll finally be able to have the app running!