Make the class path validator regexp stricter.

This protects against jars that might have a 2.x.y version number that looks like
a Scala version, but it's not. We check now that the third digit, if exists, is a
digit, not any character. (previously it would fail the aspects_2.1.0.nightly jar)
Merge pull request #291 from dragos/urgent/fix-classpath-validator

Make the class path validator regexp stricter.