Bump Error Prone Gradle Plugin version (#804)

Just to stay up to date.
diff --git a/build.gradle b/build.gradle
index a16feec..07f6250 100644
--- a/build.gradle
+++ b/build.gradle
@@ -36,7 +36,7 @@
 }
 plugins {
   id "com.diffplug.spotless" version "6.20.0"
-  id "net.ltgt.errorprone" version "3.0.1" apply false
+  id "net.ltgt.errorprone" version "3.1.0" apply false
   id "com.github.johnrengelman.shadow" version "8.1.1" apply false
   id "com.github.kt3k.coveralls" version "2.12.0" apply false
   id "me.champeau.jmh" version "0.7.1" apply false