suppress the to_s on the Client object.
- Include the URL in the output produced by Client.{inspect,to_s}.
- Fix test failures in Travis.
- Fix broken Travis test.
Merge pull request #15 from heroku/suppress-client-to-s

- `Client.inspect` and `Client.to_s` print out a short representation
  of the instance (the previous version was incredibly verbose and
  unhelpful).
- A couple of tests have been reworked to play friendly with Travis.