skip undefined fixVersions

fixVersions may be not set (undefined), e.g. on TC tickets

```
Looking up 5 issues..

TC-4613        Support Widgets and Require in ListItem Templates
Updated        2014-09-06T06:44:35.000+0000
Status         Open

TypeError: Cannot read property 'length' of undefined
    at Request._callback (/Users/manu/git/tickets/lib/tickets.js:194:41)
    at Request.self.callback (/Users/manu/git/tickets/node_modules/request/request.js:199:22)
    at Request.EventEmitter.emit (events.js:98:17)
    at Request.<anonymous> (/Users/manu/git/tickets/node_modules/request/request.js:1160:14)
    at Request.EventEmitter.emit (events.js:117:20)
    at IncomingMessage.<anonymous> (/Users/manu/git/tickets/node_modules/request/request.js:1111:12)
    at IncomingMessage.EventEmitter.emit (events.js:117:20)
    at _stream_readable.js:920:16
    at process._tickCallback (node.js:415:13)
```