[WHISKER-13] Error during site generation with Java8
- Move the version of Findbugs Maven Plugin to a property, and conditionally change the version to a newer one if running under Java 1.8+

git-svn-id: https://svn.apache.org/repos/asf/creadur/whisker/trunk@1623073 13f79535-47bb-0310-9956-ffa450edef68