Using mojo extra-enforcer-rules enforceBytecodeVersion.

This rule enforces a bytecode version in dependencies.
This would prevent cases like 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.
Using a property to share target jdk version with enforcer rule.
Merge branch 'prevent-too-recent-bytecode-in-deps' of github.com:Batmat/jenkins into Batmat-prevent-too-recent-bytecode-in-deps
#781 updates. We now require Java 6+.
If we are on Java 6 anyway, we can use the current Commons Codec.
Deleting obsolete comment.
Comment.
Merge branch 'master' of github.com:jenkinsci/jenkins