Fix version number check to deal with non-numbers

Fix for #155 

Added my own comparator, gradle doesn't have any public ones, basically a copy of one of theirs with a few modifications.

http://forums.gradle.org/gradle/topics/is-there-a-way-to-programmatically-ensure-a-minimum-gradle-version-using-non-internal-gradle-apis
does the class have any unit tests you could pull over?