Require key

Look at https://github.com/fastly/fastly-ruby/commit/90e152aa186e14a2fe2793a35e89302eeb8854e0 for functional changes.
Probably safe to remove this since it's empty and to also remove the only method that calls this in `lib/fastly.rb:78`

I say kill `set_customer` altogether since it can *only* be used by Fastly Admins and I'm of the opinion that we shouldn't muck up our "public" clients with special-for-fastly-use-only hacks.

thoughts? cc/ @alieander @ezkl @set5think