Bind max test Gradle version to current Gradle

[Will-thom]

Summary

• use GradleVersion.current() for the test DSL max supported Gradle version
• add a focused unit test for the TestVersions max Gradle version

Fixes #556.

Test

• ./gradlew.bat --no-daemon --console=plain --max-workers=1 :plugin:compileKotlin
• ./gradlew.bat --no-daemon --console=plain --max-workers=1 :plugin:compileTestKotlin
• ./gradlew.bat --no-daemon --console=plain --max-workers=1 :plugin:test --tests org.jlleitschuh.gradle.ktlint.testdsl.TestVersionsTest

[JLLeitschuh]

We do cross jdk version testing. I'm not certain what this will break. I'm running the tests to see