Downgrade to the last version of findbugs-maven-plugin that works on Java 6.

git-svn-id: https://svn.apache.org/repos/asf/creadur/whisker/trunk@1622957 13f79535-47bb-0310-9956-ffa450edef68
[WHISKER-12] Cleanup Build Infrastructure
- Add maven-enforcer-plugin to check the bytecode version of dependencies transitively

git-svn-id: https://svn.apache.org/repos/asf/creadur/whisker/trunk@1622962 13f79535-47bb-0310-9956-ffa450edef68