Update `rake push` to use 'gerrit' remote

Let's be explicit about where this is going: through gerrit, then to
Github.

Long term plan is to make Jenkins responsible for building+pushing, then
removing push perms for refs/head/master. While under active
development, it's nice to have an easy way to push to 'production',
though.

Removed --force flag since we're blocking it in gerrit, and I've added a
convenience script (parent commit) to make sure it'll be a fast-forward
build/push.

Change-Id: I0bf7c430b3e121cd61f1ad89269219ba0c9ccc76
Reviewed-on: https://gerrit.causes.com/22409
Tested-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>