Add heartbeat generator

Story: https://www.pivotaltracker.com/story/show/82210230

For monitoring purposes, a heartbeat message is sent to the published_documents exchange.
These messages have a timestamp and the hostname of the machine, along with a header of "application/x-heartbeat".

This adds a cron job running every minute to emit the heartbeat messages.
Yes. It can be made more explicit by using `Whenever.path`. Or we can use `GOVUK_APP_ROOT', provided by `govuk_setenv`