[FIX] list jobs was returning attributes and not jobs, tightened the search a little bit

Fixes #16

Prior to this patch, the `runbot_build.list_jobs()` function would return:
```
['job', 'job_10_test_base', 'job_20_test_all', 'job_30_run', 'job_age', 'job_end', 'job_start', 'job_time']
```
Now it returns
```
['job_10_test_base', 'job_20_test_all', 'job_30_run']
```