Create command for deploying with heroku pipeline plugin

[patcon]

Thinking we can do this without access control, in all honesty. This command would only deploy things to roobot-prod heroku app that are already on roobot-test. And things only get to roobot-test if they're tagged commits on "master", as that's all the travis is told to deploy.

Thinking this would pair nicely with a custom heroku buildpack that allowed us to use an envvar to set keybase.io users who must sign the tag in order for it to compile and deploy.

So basically, here's the pieces:

• set up travis to only deploy tagged commits on master to heroku
• custom heroku buildpack that expects something like this: HEROKU_BUILDPACK_SIGS=whit537,patcon,seanlinsley,<etc>
• enable heroku labs:enable user-env-compile (See: https://discussion.heroku.com/t/passing-environment-variables-to-slug-compiler/75)
• check the signed tag against keybase.io usernames from envvar, ensuring that it has been signed by an authorized user
• if everything looks good, deploy to roobot-test (ie. everything on roobot-test is cryptographically guaranteed trusted by us)
• use a hubot command to deploy a release from roobot-test to roobot-prod

EDIT: Apparently heroku labs:enable user-env-compile is enabled by default now:
http://stackoverflow.com/a/22909608/504018