Upgrade build to Gradle 2.5 for JDK 9 support

Overview

Gradle 2.5 aims to support JDK 1.9.

In fact, the Gradle 2.5 nightly build already supports building spring-test (and presumably the rest of the spring-framework) against JDK 1.9.

For details, see the comments from Adrian Kelly on the Gradle Forums.

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Update from Luke Daley – May 12, 2015

To be clear, it's not quite guaranteed that 2.5 works flawlessly with Java 9. We are making steps towards this; how far we get remains to be seen.

At this time we won't be doing a point release of 2.4. You could try using the http://gradle.org/nightly2 build in the interim.

We are looking to issue the first RC of 2.5 in the next two-ish weeks.