Added Rubocop, upgraded Faraday, removed Gemfile.lock.

This is a bunch of related things.

* Removed Gemfile.lock, this is a library it shouldn't rely on fixed locked gems, but only on its declaration in .gemspec - people using hyperclient don't get that lock-in.
* Added Rubocop, automatic style guide, cleans up common Ruby issues and nobody needs to argue about code style anymore ever.
* Replaced faraday digest implementation with https://github.com/bhaberer/faraday-digestauth.
I think this doesn't really belong in Hyperclient and is a generically useful Faraday thing - who copies from whom is beyond the point, no?