Update version to 4.0.0.dev and try fix a race condition on job reservation (refs #2)
Update travis-ci config
Fix path to CI gemfile
Add CHANGELOG file [ci skip]
Add contributors to README [ci skip]
Allow ruby-head and rbx to fail
Add a test for race condition with multiple workers trying to pick up
the same job.

- Causes an exception when trying to destroy the job after multiple
  workers have picked up the same job.