bump the zendesk_api gem

the zendesk_api gem has recently been improved with better
error handling (ie it actually raises exceptions now!). This
means that some custom code can be removed.

until the zendesk_api changes have been released, a git reference
will be used.
bump zendesk_api gem again as a RC has been pushed
another exception thrown when fallback email missing
more specific error name when zendesk doesnt create ticket
fix: correct error filename
Merge remote-tracking branch 'origin/bump-zendesk-api-gem-for-better-error-handling'