Update Gradle to 2.11 so that it can handle JSK 9 'early access' too.

[paul-hammant]

Unzipping /Users/paul/.gradle/wrapper/dists/gradle-2.6-bin/41mo8i0dq1d1kjiseo0r47d25o/gradle-2.6-bin.zip to /Users/paul/.gradle/wrapper/dists/gradle-2.6-bin/41mo8i0dq1d1kjiseo0r47d25o
Set executable permissions for: /Users/paul/.gradle/wrapper/dists/gradle-2.6-bin/41mo8i0dq1d1kjiseo0r47d25o/gradle-2.6/bin/gradle

FAILURE: Build failed with an exception.

* What went wrong:
Could not determine java version from '9-ea'.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.