Release v1.1.1-rc

The release candidate does some much-needed spring cleaning! 🌬️
Gringotts is now totally (compile-time) warning free (upto elixir
1.6.5) :yay:

Core
----
Remove our implementaion of the `Gringotts.Money` protocol as ex_money
implements (and tests!) it for us.
Updated various docs and cleaned up some code smell.

Gateways
--------
1. Refactor `Paymill`, makes it much more readable and also completes
   all the pending functionality.
2. Remove/Deprecate `Wirecard` as it does not meet the standards of
   this library.

docs: add changelog for 1.1.0

Release notes:-

* Trexle payment gateway integrations
* Docs updated
* CAMS documentation

Release notes:-

* Better documentation
* Better examples
* Better coverage than before

Version 1.0.0