Changing Git API Gems.

I won't be surprised if this needs fixing to pass CI, but IWOMM as best
I can tell, now that `pastel` 0.4.0 has been released and the code does
what the documentation has *said* it does (`#detach`) for some time.

Aruba was something I thought I'd need before I stumbled across RSpec
3's `output(…).to_stdout` matcher. Win!

The `github_api` Gem doesn't support things like cloning, and the
answer its author gave when asked was… a good working definition of
"highly suboptimal".

RSpec: 15 examples, 0 failures
RuboCop: 12 files inspected, no offences detected