Using non-semantic version numbers causes errors

Version numbers must be entered in the format x.x.x, or the initial build eventually fails after generating scaffolding.

Failure is a bit confusing from a user perspective: files are generated but the `src` folder and the `gruntfile.js` are left in the folder. The theme file that's generated is actually still functional.

This is mega confusing to non-technical users (see: myself!)

If nothing else, entering a non-copasetic value here should yield an immediate error, rather than a confusing one later down the line.