Investigate usage of official heroku-php-buildpack

[josegonzalez]

I got it working by setting the following in a Procfile:

web: vendor/bin/heroku-php-apache2 webroot

Nginx didn't work, though there is no reason it shouldn't. Haven't had time to investigate thoroughly, so filing an issue to make sure this doesn't get lost.

[chrisvogt]

I'm submitting a PR for this right now. The CHH buildpack does not seem to be working for CakePHP anymore and is abandoned at the moment.

The changes in 74ce0ec get this app template working again, and I've updated the README documentation for the official buildpack and added instructions on setting the necessary DATABASE_URL config variable.

[HarderWork]

Redis isn't part of the buildpack. Solved by adding it to the require-section of composer.json:
"ext-redis" : "*"
File-engine is used by default in app-template, but makes no sense on an isolated heroku dyno, so maybe it should be mentioned in the heroku-section of the readme?