Fix the jobs flag to not break with older bundler

```Systems using bundler version 1.4 or earlier do not have the -j flag availabe
for the bundle command.  To prevent this, we need to default the -j flag to not
be present and only enable it if the user explicitly declares
gitlab_bundler_jobs.```

Implemented with your suggestion, @sbadia.  It still doesn't feel good, but if we want to support older bundler versions, I do not see another good way.  Fixes #155