Pessimistic version constraint for beefcake

beefcake 0.4.0 introduced mutation of strings, which causes severe problems if you're dealing with frozen strings (like in riemann-tools; which is now completely broken on any modern ruby as a result). Not sure as to whether we should try to fix this in riemann-tools or in beefcake, but I believe this pull request is a good measure regardless to prevent future breakages.