Fix Rspec 2.99.0.beta2 and above

As of Rspec version 2.99.0.beta2, 's' is deprecated as an alias for the
documentation formatter[1].

Use long arguments to specify the documentation formatter so that it's
clearer what these arguments are. `-c` also becomes `--color`.

[1]: https://github.com/rspec/rspec-core/releases/tag/v2.99.0.beta2
Add `delete_exchange` method

And its accompanying tests, modelled on the `delete_queue` method.
Merge pull request #7 from alphagov/delete-exchange

Add `delete_exchange` method and fix Rspec 3 errors