ACCUMULO-3405 Drop findbugs-maven-plugin to 2.5.5

  Drop findbugs-maven-plugin down to latest version that runs on Java6.
  FindBugs2 (which is used by version 3 of the plugin) requires Java7.

  This only applies to the 1.6 branch. Merging forward with -sours.
Merge branch '1.6'

  Merging with -sours for ACCUMULO-3405