Update Gradle to 6.0, support Java 13 (#180)

Upgrade Gradle to 6.0.
* This is needed as Gradle 5.x did not support Java 13
* Jacoco needed a version upgrade to support Java 13
* Upgrade Errorprone to fix some breaks
* Another Errorprone check is broken in 13
* The 'Finally' Errorprone check is only broken in Java 12
* JMH needed a version upgrade and a new build flag
* Switch to bin distribution for smaller builds

Signed-off-by: Danno Ferrin <danno.ferrin@gmail.com>

build.gradle

@@ -23,8 +23,8 @@ plugins {
   id 'com.github.ben-manes.versions' version '0.26.0'
   id 'com.github.hierynomus.license' version '0.15.0'
   id 'io.spring.dependency-management' version '1.0.8.RELEASE'
-  id 'me.champeau.gradle.jmh' version '0.4.8' apply false
-  id 'net.ltgt.errorprone' version '0.8.1'
+  id 'me.champeau.gradle.jmh' version '0.5.0' apply false
+  id 'net.ltgt.errorprone' version '1.1.1'
 }
 
 if (!JavaVersion.current().java11Compatible) {
@@ -98,7 +98,7 @@ allprojects {
   version = rootProject.version
 
   jacoco {
-    toolVersion = '0.8.2'
+    toolVersion = '0.8.5'
     if (project.tasks.findByName('referenceTests')) {
       applyTo referenceTests
     }
@@ -197,9 +197,13 @@ allprojects {
       check('FieldCanBeFinal', CheckSeverity.OFF)
 
       // This check is broken in Java 12.  See https://github.com/google/error-prone/issues/1257
-      if (JavaVersion.current().java12Compatible) {
+      if (JavaVersion.current() == JavaVersion.VERSION_12) {
         check('Finally', CheckSeverity.OFF)
       }
+      // This check is broken after Java 12.  See https://github.com/google/error-prone/issues/1352
+      if (JavaVersion.current() > JavaVersion.VERSION_12) {
+        check('TypeParameterUnusedInFormals', CheckSeverity.OFF)
+      }
 
       check('InsecureCryptoUsage', CheckSeverity.WARN)
       check('WildcardImport', CheckSeverity.WARN)
@@ -424,6 +428,7 @@ subprojects {
       include = _strListCmdArg('include', [''])
       humanOutputFile = project.file("${project.buildDir}/reports/jmh/results.txt")
       resultFormat = 'JSON'
+      duplicateClassesStrategy = 'warn'
     }
 
     dependencies { jmh 'org.apache.logging.log4j:log4j-api' }

gradle/wrapper/gradle-wrapper.properties

@@ -2,4 +2,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.4.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.0-bin.zip