Update rufus-scheduler to ~> 3.0

Update `rufus-scheduler` gem to require a version which is greater (or equal) to 3.0 and less than 4.0.
Moving to newer versions of `rufus-scheduler` should also solve #225.

Note, some tests are failing, but I need help on solving them.

EDIT: Two tests are failing only in my local machine. I'm on a Mac. The two tests are:

- test_writes_pid_to_pidfile_when_given_and_cleans_up_automatically
- test_writes_pid_to_pidfile_when_given_and_cleans_up_manually