Upgraded to new travis D build system

This upgrades the Travis build system to use the native D support, instead of installing compilers manually. It removes  fair amount of code, and should make maintenance much easier.

A sample of what the builds look like is available on my fork of dub, [here](https://travis-ci.org/ColdenCullen/dub).
It was set to just 2.066.0 before, and I didn't want to remove that as part of this PR. If you don't think we need it, I can get rid of it.