Updated Tests, Use Guzzle

I updated some old unit tests which failed due to the port being changed over to :80 from :8080 previously.

I also, mainly, ported the ApiClient over to use Guzzle, which is faster (curl-based with file_get_contents). Additionally, with that, I re-factored some existing code which was a bit shaky (namely the manual creation of a JSON string).

In order to test that Guzzle was working correctly, I removed the mocks from the test. The mocks did not actually test the request itself. If you want to reinstate them I'd suggest using Mockery and inserting a Guzzle client mock into the constructor of the ApiClient class - that'll give you the best results.

(by the way, I just want to thank you for having written one of the only decently coded PHP classes I've seen related to Minecraft stuff)