Figure out the way to test the plugin with latest Gradle versions

[mlopatkin]

It is important for us not to break plugin compatibility with the newer Gradle versions. There are two sources of changes:

• Gradle constantly evolves
• The plugin may evolve as well

This somewhat requires a two-way synchronization between the two.

As of today, we have supported Gradle versions hardcoded in the plugin test suite, which requires manual update. It probably isn't ideal.