Don't use a read-only field in Zd ticket creation

According to the Zendesk changes roadmap[1], the description field
on tickets is read-only and shouldn't be used during ticket creation.
The 'comment' field should be used instead.

[1]: http://developer.zendesk.com/documentation/rest_api/changes_roadmap.html#january-2,-2014,
Merge pull request #125 from alphagov/dont-use-read-only-field-in-zendesk-ticket-creation

Don't use a read-only field in Zd ticket creation