Update the errorprone library in order to build with gradle out of the box. (#58)

Author: timduly4

build.gradle

@@ -35,7 +35,7 @@ repositories {
 
 dependencies {
     errorproneJavac("com.google.errorprone:javac:9+181-r4173-1")
-    errorprone("com.google.errorprone:error_prone_core:2.3.1")
+    errorprone("com.google.errorprone:error_prone_core:2.4.0")
     compile group: 'com.uber.cadence', name: 'cadence-client', version: '3.5.0'
     compile group: 'commons-configuration', name: 'commons-configuration', version: '1.9'
     compile group: 'ch.qos.logback', name: 'logback-classic', version: '1.2.3'