[NOTICE] PreCommit now running against Hadoop3 artifacts successfully

[Josh Elser]

tl;dr You may start seeing some new "failures" WRT Hadoop3 compatibility 
with PreCommit builds. They are (very likely) true negatives.

Sean, Appy, and myself think we got to the bottom of why our PreCommit 
had let (at least two) patches that broke Hadoop3 compatibility on 
HBASE-17441. From what I can tell so far, there haven't been too many 
changes upstream, but thought it would be good to give a heads up to 
folks (lest someone wastes time thinking it's just PreCommit being flaky).

- Josh