Add a Heroku Button for easy deployments

[jvandyke]

https://blog.heroku.com/archives/2014/8/7/heroku-button

The only odd this is that the app's URL needs to be set after it's deployed, unless you know you'll be using a DNS and know the address. Not so helpful for quick test deployments.

[iamcal]

We only really need the URL for cron-style invocations, since we can fetch it out of the environment for web requests. An alternative could be detecting it at the first web invocation and stashing it in the DB (same for cookie settings).

[jvandyke]

Good to know. The URL param is optional for this PR, so it shouldn't make a difference. Anything you want changed?

[dzuelke]

Change the buildpack URL and remove that #redis branch, it should work just fine with the official buildpack.

Instead, add a composer.json like the following:

{
  "require": {
    "ext-redis": "*"
  }
}

That will activate the redis extension on deploy.