Back to green on CircleCI

[Johan Engelen via digitalmars-d-ldc]

LLVM's APT repo is back on track and up-to-date with trunk --> 
CircleCI is green again!

I hope we can keep it green by updating AppVeyor's LLVM package 
whenever LLVM trunk has an API breaking change. I.e. whenever 
CircleCI starts to fail compilation because of an LLVM change: 
update AppVeyor's package and fix the LLVM3.9 compile. I'm often 
available to start a new AppVeyor package build, so ping me if it 
is needed.

cheers,
   Johan