rest_client not required if using Client without CLI

[danielmorrison]

In 89293ef require 'rest_client' was moved out of many classes and into cli.rb.

Unfortunately, if you try to use Heroku::Client without the Heroku::CLI (which is what kjohnston/pgbackups-archive does) you get an error that RestClient hasn't been required:

NameError: uninitialized constant Heroku::Client::Pgbackups::RestClient
    from /app/vendor/bundle/ruby/2.1.0/gems/heroku-3.10.3/lib/heroku/client/pgbackups.rb:14:in `authenticated_resource'

It seems like Heroku::Client should also require 'rest_client'. I'm happy to whip up a PR for that, but wanted to see if there's a better solution first.

Cheers!

[jdx]

that sounds like a good solution

[jdx]

removing that require fixed a bunch of problems, so we definitely want to keep the commit