Upgrade guava from 18 to 20

[davidxia]

fixes #826

[davidxia]

just testing do not merge

[codecov-io]

Codecov Report

Merging #849 into master will not change coverage.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master     #849   +/-   ##
=========================================
  Coverage     66.37%   66.37%           
+ Complexity      743      742    -1     
=========================================
  Files           166      166           
  Lines          3138     3138           
  Branches        357      357           
=========================================
  Hits           2083     2083           
  Misses          898      898           
  Partials        157      157

[mattnworb]

Would be nice to start relocating the guava classes before we do another upgrade, as this sort of change has a big effect on all downstream users of this and/or helios-client, helios-testing etc. See spotify/helios#1119 for instance.

[mattnworb]

Also I wonder if after we upgrade this library to require Java 8 if we could ditch guava altogether, as it is a large dependency to be dragging into user's projects.

[davidxia]

Closing as there are already two other PRs that do what this one does. #792 upgrades to guava 20, and #850 shades and relocates guava.