Fix javadoc error (#1832)

* Update Lombok for fix to [JDK12 delombok issue](projectlombok/lombok#2082)
Also upgrade Gradle Lombok plugin

build.gradle

@@ -1,6 +1,6 @@
 plugins {
     id 'com.gradle.build-scan' version '1.16'
-    id 'io.franzbecker.gradle-lombok' version '1.14'
+    id 'io.franzbecker.gradle-lombok' version '3.1.0'
     id 'nebula.provided-base' version '3.0.3'
     id 'com.github.johnrengelman.shadow' version '2.0.2'
     id "com.jfrog.bintray" version "1.8.4" apply false
@@ -21,7 +21,7 @@ subprojects {
     compileTestJava.options.encoding = 'UTF-8'
 
     lombok {
-        version = '1.18.6'
+        version = '1.18.8'
     }
 
     task delombok(type: io.franzbecker.gradle.lombok.task.DelombokTask) {