all: Enable ErrorProne during compilation

ErrorProne provides static analysis for common issues, including
misused variables GuardedBy locks.

This increases build time by 60% for parallel builds and 30% for
non-parallel, so I've provided a way to disable the check. It is on by
default though and will be run in our CI environments.
diff --git a/.travis.yml b/.travis.yml
index 3022774..971833d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,6 +22,7 @@
   - mkdir -p $HOME/.gradle
   - echo "checkstyle.ignoreFailures=false" >> $HOME/.gradle/gradle.properties
   - echo "failOnWarnings=true" >> $HOME/.gradle/gradle.properties
+  - echo "errorProne=true" >> $HOME/.gradle/gradle.properties
 
 install:
   - ./gradlew assemble generateTestProto install