Forge builds should be less restrictive on the JDK version used

The enforcer plugin forbids usage of newer JDKs. It is ok to build Forge with newer JDKs as long as we don't use any new features.

Using the animal-sniffer maven plugin may be a solution(http://mojo.codehaus.org/animal-sniffer-maven-plugin/) since it contains rules to check if a specific feature of newer JDKs is being used in the source code.