Continuous deployment via Travis

https://www.pivotaltracker.com/story/show/46678999

I've been following the instructions here:

http://about.travis-ci.org/blog/2013-07-09-introducing-continuous-deployment-to-heroku/
http://about.travis-ci.org/docs/user/deployment/heroku/

and I *totally had it working* for a while:

https://travis-ci.org/pozorvlak/growstuff/builds/10897427

... but then I had to change my API key and it stopped working.

Anyway, this PR is not yet ready to merge. Here's what needs to be done:

 - we need to change the API key to one that works, and preferably to one associated with the Growstuff project
 - we need to change the `app` setting to

        app:
          master: growstuff-prod
          dev: growstuff-dev

For work on this, the `travis` gem is somewhere between strongly advised and essential, as it allows you to trigger new builds with the command `travis sync`.