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'm -1 on this since this gem seems a blatant rip of the actual code at hyperclient. See https://github.com/codegram/hyperclient/commit/a47d7db539367180c3093d7abf5cc1c0668013a3 and https://github.com/bhaberer/faraday-digestauth/commit/d41137a9a7809ab0e3c6107b0192f534351cb189 

Also, the gem _author_ forgot to copy the tests.