bugfix to correct the model mapping for the single task endpoint

@guidow, please take another look.  The original implementation looks like the lookup on the job model was a typo (or I'm misunderstanding something)?

In any case, this pull request should fix the problem I encountered at commit pyfarm/pyfarm-agent@a51c92b1726a781697fe1e2f7fd72c5868d5bfd3 which was producing a traceback when I tried to POST `last_error` for a specific task from the agent to the master:

```
....
File "virtualenv/pyfarm-master3/lib/python3.4/site-packages/Flask-0.10.1-py3.4.egg/flask/views.py", line 149, in dispatch_request
    return meth(*args, **kwargs)
  File "./pyfarm/master/api/jobs.py", line 914, in post
    type = Job.types().mappings[name]
KeyError: 'last_error'
```