Relax rake gem constraint from <=11.x to <=12.x.
Relax rake gem constraint from <=11.x to <=12.x.

Rake 12 has been released for quite some time. Given this gem doesn't
depend on `rake` per se (it is only required to avoid installing the
`ffi` gem on non-Windows environments), relax the constraint.

To prevent accidental breakage due to a new Rake release, we'll still
pin to < 13 for now.

jarib/childprocess,enkessler/childprocess