[scripts/zendesk] Fixes z_url error

Hi,

I had the following error trying to use the zendesk script:
```
Hubot> [Fri Sep 06 2013 14:23:17 GMT+0100 (BST)] ERROR ReferenceError: z_url is not defined
  at /Users/Pierre/code/enablers_hubot/scripts/zendesk.coffee:92:19, <js>:90:90
  at /Users/Pierre/code/enablers_hubot/scripts/zendesk.coffee:50:9, <js>:33:14
  at IncomingMessage.<anonymous> (/Users/Pierre/code/enablers_hubot/node_modules/hubot/node_modules/scoped-http-client/lib/index.js:70:20)
  at IncomingMessage.EventEmitter.emit (events.js:117:20)
  at _stream_readable.js:920:16
  at process._tickCallback (node.js:415:13)
```
This commit fixes the error (tested on my hubot).

Cheers,

Pierre,