For consistency, use `job_dir` helper in `SpiderState` extension.

The `SpiderState` extension is the only place not using the `job_dir`
helper.

This might cause an error in case JOBDIR is a nested path which does not
exists, although this is least likely by default given the job dir being
set up earlier.
Merge pull request #805 from darkrho/spiderstate-jobdir

For consistency, use `job_dir` helper in `SpiderState` extension.