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).
Well it is up to @s-ludwig but I think it should be only latest point release for each supported compiler version - I can't imagine anyone intentionally using version with more bugs.