siasia/xsbt-web-plugin — pull request #95 by cdow (2012-10-14 22:08:53+00:00)
Title: 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