Unlock jobs after worker crash - with ruby 1.8.7 compatibility for DJ V3

I needed [Ivan Vanderbyl's](https://github.com/ivanvanderbyl) patch, but was on V3, and ruby 1.8.7, so I've shameless stolen his fantastic work and have it working in DJ V3.  There've been a number of folks who have run into the problem, so figured I'd try and raise awareness a bit

Here was Ivan's [original pull request](https://github.com/collectiveidea/delayed_job/pull/130)