Changed artifact versioning to use default SBT plugin versioning

The previous artifact versioning did not support binary compatible versioning introduced in SBT 0.12.  As a result versions of this plugin built for for SBT 0.12.0 would not be automatically used by projects
using SBT 0.12.1.

I assume that the custom versioning is left over from when SBT had issues publishing plugins in Maven repos.  I believe this has since been fixed.  Let me know if you have issues