Set default compiler to be the same that built dub

Raising this PR as a bug - there are likely other things that need to change too, such as the help message.
``"  dmd (default), gdc, ldc, gdmd, ldmd"``

It would be nice if the default compiler dub used was the same that built the application.  So for me, I don't need to specify --compiler=gdc explicitly every time.