MINOR: Upgrade to Gradle 4.8.1 (#5268)

Maven Central dropped support for all versions but
TLS 1.2, so dependency resolution fails if Gradle
builds run with JDK 7. 2.0 and trunk require JDK 8,
but every other version is affected. Gradle 4.8.1
fixes the issue by enabling TLS 1.2 by default even
when JDK 7 is used.

Reviewers: Rajini Sivaram <rajinisivaram@googlemail.com>

build.gradle

@@ -68,7 +68,7 @@ allprojects {
 }
 
 ext {
-  gradleVersion = "3.0"
+  gradleVersion = "4.8.1"
   buildVersionFileName = "kafka-version.properties"
 
   maxPermSizeArgs = []