Set implicit R dependency explicitly

[MichaelChirico]

I am trying to use {rcmdcheck} for our "ancient" tests on R 3.2.0+, but it doesn't work because some of {rcmdcheck}'s upstream dependencies themselves now depend on higher versions of R:

Dependency	R dependency
callr	3.4
cli	3.4
desc	3.4
pkgbuild	3.5
with	3.5
proccesx (via xopen)	3.4

Obviously all the better if those upstream dependencies could allow an even older version of R to be tested in CI, but at least declaring {rcmdcheck} Depends: R (>= 3.5.0) would make for easier failure messaging.