Send Kernel.lambda for ->() {}.

This is an interim fix for https://github.com/rubinius/rubinius/issues/2990.

After I lock down gem versions for 2.2.7, I'll fix this with a send to an
internal lambda like MRI does so that changing Kernel.lambda won't break
-> () {} lambdas.
Removed Melbourne19.
Bumped version to 2.1.2.