ACCUMULO-3100 Pull back Guava dependency from 15.0 to 14.0.1.

Hadoop-2 still depends on 14.0.1 and uses some classes that do
not exist in >14.0.1. As such, if we depend on a newer version of
Guava, we're incorrectly advertising that Accumulo fully works
with this version: specifically tests that level MiniDFSCluster
and, specifically, clients who use MiniAccumuloCluster are subject
to failure.