Ticket epic for automating changelog generation

EDIT 31 Jul 2017: It's become clear that our previous approach isn't going to work out so we're going to ticket a new epic of work for this

Add the necessary Rake task and gem dependencies to puppetlabs-spec-helper so that we can use it to start auto generating changelogs for module releases. Because an upstream change hasn't made it into the official github-changelog-generator gem, we have to rely on a forked repo that includes the change.

Because gemspec doesn't support git (which we need to use to install our fork of the gem), we can't just put the gem straight into puppetlabs-spec-helper so it has to go in modulesync and be included in all the synced modules.