Update Webmock version and Rummager tests

* Updates Rummager tests that relied on Webmock's previous handling of
  square bracket params in urls to improve readability
* Updates and pins Webmock to >= 1.19 (because the updated Rummager tests don't
  pass with older version of Webmock)
Merge pull request #221 from alphagov/correct-rummager-tests

Update Webmock version and Rummager tests