Limit configuration cache testing to the latest version of Gradle

[wilkinsona]

For every version of Gradle in our test compatibility matrix, we test both with and without the configuration cache enabled. This doubles the number of tests for several classes for little benefit as we rarely, if ever, find bugs that are specific to the configuration cache. Given that Gradle does not enable the configuration cache by default and that it has improved significantly in the most recent versions of Gradle, we feel that it's overkill to test with it enabled as much as we do. We're going to save some resources and reduce build times by limiting our configuration cache testing to the latest version of Gradle.