Upgrade to findbugs-maven-plugin 2.5.4

This also upgrades FindBugs to 2.0.3.  Notably this improves
compatibility with Guava constructs.  Changelogs:

https://jira.codehaus.org/secure/ReleaseNote.jspa?projectId=11701&version=19833
http://findbugs.sourceforge.net/Changes.html