MINOR: Upgrade to Gradle 4.2

It includes the usual performance improvements, but
the nicest improvement for me is that the findBugs
plugin no longer outputs 10000+ lines in
Jenkins builds:

https://docs.gradle.org/4.2/release-notes.html#findbugs-plugin-does-not-render-analysis-progress-anymore

Author: Ismael Juma <ismael@juma.me.uk>

Reviewers: Rajini Sivaram <rajinisivaram@googlemail.com>

Closes #3923 from ijuma/gradle-4.2

build.gradle

@@ -77,7 +77,7 @@ allprojects {
 }
 
 ext {
-  gradleVersion = "4.1"
+  gradleVersion = "4.2"
   buildVersionFileName = "kafka-version.properties"
 
   maxPermSizeArgs = []