It's confusing that we print "1.8" and "1.9" in the build warning now that the releases are referred to as "8" and "9".
Consider changing the warning to:
gradle.rootProject.logger.error("\nWARNING:\nJava ${JavaVersion.VERSION_1_9.getMajorVersion()} is required to build the driver: " + "you are using Java ${JavaVersion.current().getMajorVersion()}.")
Which will print:
Java 9 is required to build the driver: you are using Java 8.