[NOTICE] dependency update for 1.9-SNAPSHOT

[Sean Busbey]

just a quick heads up that folks should take a look at AVRO-1975 if they are 
concerned with our dependency set. In particular, our Hadoop version is going 
from 2.5.1 -> 2.7.3 and Protobuf goes from 2.5.0 -> 2.6.1.