Nupkg artifact versions don't get incremented.

[adamskt]

[BenjaminMichaelis]

This is fixed by #38. Versions are now determined by release tag that is tagged in the format "v0.0.0", so if we want to make a 1.0.0 release we create a release on github with the tag v1.0.0 and this will upload a new artifact to nuget with the correlated version 1.0.0