Group heroku's buildpacks together
Consistently add .git for git URLs
Add missing heroku's default buildpacks

Probably makes sense to have all of Heroku's [default
buildpacks](https://devcenter.heroku.com/articles/buildpacks#default-buildpacks)
available in Dokku.

Added:

- gradle
- grails
Merge pull request #67 from amaltson/missing-heroku-default-buildpacks

Add missing heroku's default buildpacks