PR to remove magic numbers

There are plenty of sourceCompatibility and targetCompatibility with hardcoded numbers.

Can I make a PR to use the JavaVersion class?

Hmm, I don’t have strong feelings about this other than "1.6" is typically easier to digest than JavaVersion.VERSION_1_6 when reading a build script and is more in-line with a declarative DSL.

@mark_vieira Alright. I was thinking that the hard coded numbers were in there before the JavaVersion class was created.

This is likely the case although I think relying on enum coercion in build scripts is still more idiomatic.