Added test gemfiles for sequel/rails 3/4

There's probably a better way to do this, but it seems to work.
Just do 25 runs, jruby runs took too long on travis. :(
Use longer test timeout on travis, see if that explains the failures.
don't be dumb
Remove test Gemfile.lock's
Be just a bit safer.
Be explicit about using smallint for priority, though I think it probably doesn't matter.
Merge branch 'multiple_gemfiles' of https://github.com/joevandyk/que
Rename Job.queue to Job.enqueue. The word 'queue' is used in too many contexts, this is clearer. Job.queue still works, though.
Rename @default_priority and @default_run_at job variables to @priority and @run_at, for consistency.
Update the changelog to cover the recent API changes.