Fix #14 - Add ability to choose Gradle version

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](https://github.com/johnrengelman/gradle-test-kit/blob/master/src/main/java/org/gradle/testkit/functional/GradleRunnerFactory.java), but I think it is not needed when `IntegrationSpec` is used (not  GradleRunnerFactory directly).

What do you think about that?
There could be an error when `gradleVersion` and `useToolingApi = false` are used together, but I don't know if it is worth to complicate the code.