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).
That seems like a fair enough point. I'll update the PR. If @s-ludwig would rather have both I can just put it back.