Remove curb-fu once and for all

Related: https://github.com/fastly/fastly-ruby/pull/41

Curb does indeed suck. Much of the complexity around the client/fetcher stuff is due to it. 

This PR removes curb-fu entirely from fastly-ruby and adds unit tests around fastly/client.rb

Changes I saw fit to make in the process:
- Add KeyAuthRequired over just using AuthRequired
- GET 404s no longer return nil and instead raise a `Fastly::Error`
- Fixes stats_test.rb. I don't even know what was going on there.

This also makes #36 irrelevant