Pin test deps to allow for testing on Ruby 2.5/2.6
Pin test deps to allow for testing on Ruby 2.5/2.6

This has no impact on the actual app, but it's making our tests fail

Signed-off-by: Tim Smith <764ef62106582a09ed09dfa0b6bff7c05fd7d1e4@chef.io>

chef-customers/knife-tidy