compatibility with recent dub versions

It now refers to specific versions for dub. Also I added an explicit importPath to make it clear where the import tree starts.

The gl3n dependency will need tagging with an explicit release too, but that would require action on https://github.com/Dav1dde/gl3n/issues/42
Merge pull request #16 from John-Colvin/patch-1

compatibility with recent dub versions