Fix broken specs

[subinwalter]

Issue

request_spec is failing with

Mocha::NotInitializedError:
       Mocha methods cannot be used outside the context of a test

Fix

• Remove hard-coded faraday version
• Remove stub and define methods in Class.new block, thereby removing the need for mocha
• Remove mocha

[joesiewert]

@subinwalter 4.5 years later I'm running into the very same Mocha issue on this gem. Your fix here got me unblocked! 🙇