Upgrade to Gradle 4.8.1

Includes Kotlin 1.2.50, RxJava 2.1.15 and Protobuf 3.6.

Issue: SPR-16475

build.gradle

@@ -11,7 +11,7 @@ buildscript {
 // 3rd party plugin repositories can be configured in settings.gradle
 plugins {
 	id "io.spring.dependency-management" version "1.0.5.RELEASE" apply false
-	id "org.jetbrains.kotlin.jvm" version "1.2.41" apply false
+	id "org.jetbrains.kotlin.jvm" version "1.2.50" apply false
 	id "org.jetbrains.dokka" version "0.9.17"
 	id "org.asciidoctor.convert" version "1.5.7"
 }
@@ -37,13 +37,13 @@ ext {
 	junitPlatformVersion = "1.2.0"
 	junitJupiterVersion  = "5.2.0"
 	junitVintageVersion  = "5.2.0"
-	kotlinVersion        = "1.2.41"
+	kotlinVersion        = "1.2.50"
 	log4jVersion         = "2.11.0"
 	nettyVersion         = "4.1.25.Final"
 	reactorVersion       = "Californium-BUILD-SNAPSHOT"
 	rxjavaVersion        = "1.3.8"
 	rxjavaAdapterVersion = "1.2.1"
-	rxjava2Version       = "2.1.14"
+	rxjava2Version       = "2.1.15"
 	slf4jVersion         = "1.7.25"	  // spring-jcl + consistent 3rd party deps
 	tiles3Version        = "3.0.8"
 	tomcatVersion        = "9.0.8"
@@ -283,7 +283,7 @@ configure(rootProject) {
 
 	wrapper {
 		description = "Generates gradlew[.bat] scripts"
-		gradleVersion = '4.8'
+		gradleVersion = '4.8.1'
 
 		doLast() {
 			def gradleOpts = "-XX:MaxMetaspaceSize=1024m -Xmx1024m"

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.8-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.8.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

spring-web/spring-web.gradle

@@ -54,7 +54,7 @@ dependencies {
 	optional("com.fasterxml.jackson.dataformat:jackson-dataformat-smile:${jackson2Version}")
 	optional("com.fasterxml.jackson.dataformat:jackson-dataformat-cbor:${jackson2Version}")
 	optional("com.google.code.gson:gson:2.8.5")
-	optional("com.google.protobuf:protobuf-java-util:3.5.1")
+	optional("com.google.protobuf:protobuf-java-util:3.6.0")
 	optional("com.googlecode.protobuf-java-format:protobuf-java-format:1.4")
 	optional("com.rometools:rome:1.10.0")
 	optional("com.caucho:hessian:4.0.51")