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.

Without this change, Zendesk ticket creation will stop working in early Feb.

[1]: http://developer.zendesk.com/documentation/rest_api/changes_roadmap.html#january-2,-2014,