prevent accidentally pushed tags from breaking versioning

[Schipy]

would be nice to add protection by filtering for X.Y[.Z] format tags in git describe, like git descirbe --tags --match [0-9]*[.][0-9]*([.][0-9]*)+

[lptr]

Versioning is not handled here, but in the Prezi plugin. It already checks for tags starging with a number. I don't think the rule should be more strict than this.

https://github.com/prezi/gradle-prezi-plugin/blob/master/src/main/groovy/com/prezi/gradle/PreziPlugin.groovy#L106