Update compiler plugin and error-prone to latest versions

Update:

  - maven-compiler-plugin to 3.6.1
  - error_prone_core to 2.0.15
  - plexus-compiler-javac-errorprone to 2.8.1

Remove test statements that cause "[SelfEquals] An object is tested
for equality to itself" error [1] from error-prone.

[1] http://errorprone.info/bugpattern/SelfEquals

Change-Id: Idbc027711724ae4cbba25118c98451c8ebde6a22