Update zendesk_api to the latest gem version

The upgrade in the 'zendesk_api' gem exposed some bugs in the
gds_zendesk test helpers, which were causing failing tests.

Specifically, the 'zendesk_api' library expects the response from
the Zendesk API to have content type of 'application/json' and the webmock
stubs used to simulate the Zendesk API didn't set that explicitly.
Merge pull request #15 from alphagov/update-zendesk_api-gem

Update zendesk_api to the latest gem version