Cron daemon uses 2.x version of rufus-scheduler

With the >= 2.0.3 version lock bundler will install a 3.x version of
rufus-scheduler. Using this version of rufus gives me errors. So I'm
locking the gem to a 2.x version of rufus-scheduler.
Merge pull request #79 from IanWhitney/master

Cron daemon uses 2.x version of rufus-scheduler