Using mojo extra-enforcer-rules enforceBytecodeVersion.

This rule enforces a bytecode version in dependencies.
This would prevent cases like [JENKINS-8914](https://issues.jenkins-ci.org/browse/JENKINS-8914) where someone had
introduced a dependency compiled with JDK1.6 at a time where
Jenkins was still on 1.5.

With 1.7 already here and 1.8 coming, this should help no to do that
again.