Stop running Travis tests using LLVM 3.0.

Sets the stage for removal of 3.0 support.
Removed LLVM 3.0 compatibility code.

There might be still some pieces left here and there, and
there is certainly code that could be rewritten in a nicer
way with the 3.0 requirement out of the picture.
Removed LLVM support from druntime as well.
Merge pull request #374 from klickverbot/remove-3.0

Remove LLVM 3.0 support