export correct function from buildbot.util

The commit b4fb9fbf2243db6d091202f58ced453483fecd29 introduced
buildbot.util.diffSets() function, however it was listed as
'diffLists' in the __all__ list. Use correct name.
remove non-existent symbol from __all__

The buildbot.util.LRUCache class have been removed in
a389b086e65ac8107ae6fdd288dc51a9854a992c commit. Remove it from __all__
list as well.
enable undefined-all-variable (E0603) pylint check
Merge branch 'pylint-undefined-all-variable' of git://github.com/elmirjagudin/buildbot