Use `bundle exec` when executing clockwork

Starting the Cronut server in development if you have a newer version of
clockwork on your system than Cronut is using results in an error unless you use
`bundle exec foreman start`, which then requires you to have foreman in your
`Gemfile`. This change treats `clockwork` the same way `rails server` gets
handled when the Procfile processes are started.
Merge pull request #4 from cortexintel/use-bundle-exec-for-clockwork

Use `bundle exec` when executing clockwork