I would like to upgrade to Gradle version 2.10 so that we can continue testing with JDK9 pre-releases.

Java 9 since build 95 includes "Project Verona", an initiative to rationalize, standardize and cleanup the versions reported by the JVM. This does unfortunately break some build tools: specifically Gradle was fixed for this in version 2.10.

In other words: when using Gradle versions <2.10 with recent builds of Java 9 the build is broken.