Add support for test optimization

I've added support for [test optimization](https://confluence.atlassian.com/display/CLOVER/About+Test+Optimization) to the plugin. This involved some other small changes, like upgrading the Gradle wrapper, tweaking the existing tests to pass on Windows machines, and fixing a bug that was preventing Clover from recognizing tests in Groovy projects.

I've also added some integration tests that run the plugin on actual Gradle projects.

The project now nominally depends on Clover 3.1.7, but in actuality the test optimization test requires the as-yet-unreleased Clover 3.1.8. The reason is that Clover 3.1.7 and prior had a bug in the test optimization logging code that prevented logs from being emitted if test optimization was used outside of an Ant task (which this code does).