Update rufus-scheduler to ~> 3.0

Update rufus-scheduler gem to require a version which is greather (or
equal) to 3.0 and less than 4.0.

Note, some tests are failing, but I need help on solving them so I'm
commiting this anyway.
Addressing warning from redis-namespace related to `#unwatch`
Merge pull request #426 from resque/update-rufus-to-3.0

Update rufus-scheduler to ~> 3.0