Enable errorprone check to ensure hashCode is implemented.

Bug:
Change-Id: I918fd87f5d0175cdff928a8be64c6686d6689dfa
diff --git a/build.gradle b/build.gradle
index ef09f76..e189e57 100644
--- a/build.gradle
+++ b/build.gradle
@@ -21,7 +21,8 @@
     '-Xep:RemoveUnusedImports:WARN',
     '-Xep:MissingOverride:WARN',
     '-Xep:OvershadowingSubclassFields:WARN',
-    '-Xep:IntLongMath:WARN']
+    '-Xep:IntLongMath:WARN',
+    '-Xep:EqualsHashCode:WARN']
 
 apply from: 'copyAdditionalJctfCommonFiles.gradle'