Allow clockwork to be run using the daemons gem

Clockwork ships with a helpful clockworkd executable, which uses the
daemons gem to daemonize clockwork. This provides an easy way to
start/stop/restart a deamonized clockwork process when deploying to
production servers manually. However, the daemons gem unfortunately does
funky things to the working directory that was preventing the
application from booting. Changing this `require` line to using
`require_relative` solves the issue and makes Discourse easier for Rails
devs to deploy to their own VPSes using, say, Capistrano.

Signed-off-by: David Celis <me@davidcel.is>