Specify a minimal required version of checkmate

[hongyuanjia]

assert_multi_class() was added in {checkmate} Version 1.9.0 (2019-01-09). Even this version was published 3 years ago, it is still possible that the user may have already installed an older version. Actually, there is already an error reported from a user that failed to load {epwshiftr} due to a lower version of {checkmate}