Removes unnecessary require

Because of how Bundler configures load-paths, this gem's `lib/mercury/authentication` will always supercede `mercury-rails`'s, and `mercury-rails` already handles this require in [an initializer](https://github.com/jejacks0n/mercury/blob/master/lib/mercury/engine.rb#L14).

On the flip-side, due to other load-order vagaries, this file is being required before other parts of the application have been, emitting fun `uninitialized constant` errors. Letting `mercury-rails`'s initializer handle the require renders this issue moot.