Fix #14 - Add ability to choose Gradle version

The same as #23, but for the branch `gradle-1.12`.

I implemented an ability to select which Gradle version will be used (in tooling API mode only) for testing the build. It allows to make a regression tests with different plugin version (currently due to #13 only in 1.x or 2.x).

I like the idea with configuration closure used by @johnrengelman in his fork of gradle-test-kit, but I think it is not needed when IntegrationSpec is used (and not GradleRunnerFactory directly).

What do you think about that?