Add ClearDB as a default addon

This seems reasonable. Most PHP apps will want a database and the only one that comes with Heroku and we support is the ClearDB MySQL addon. 

It doesn't delay the buildpack speed and shouldn't influence the dev (except the fact that if it's not being used it's just sitting there...)

One of the main drivers for me is [Wordpress](https://github.com/ClaudiuC/wordpress-heroku-hhvm) - it seems like an awkward step to do when comparing to deployment of Wordpress on a LAMP host.