Update compiler plugin and error-prone to latest versions
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

Change-Id: I9eb74755c28e0e23f5b43b3da10aa43f232d89eb

GerritCodeReview/gwtjsonrpc,GerritCodeReview/gwtjsonrpc