make dub use my version of dmd

[Adam D. Ruppe]

For future reference, newer dubs (v 1.17 + i think) allow 
--compiler=dmd-version for example.

You need to put the exe in your PATH and rename it yourself, but 
it recognizes *dmd-* (or *ldc2-* or *gdc-*) all the same so you 
can specifiy them.

I was doing that in early versions of my android thing, i made a 
program "android-ldc" and then did "dub build 
--compiler=android-ldc" and it all worked.

S o you can do that with versions too.