This updates logic to add a `#scheme_and_version` method to be used
with `.sort_by` and `.max_by`. Using `Keg#version` by itself can be
inaccurate when different version schemes are present. This also
updates the behavior of `Formula#eligible_kegs_for_cleanup` to match
the previous behavior. We were dropping the wrong keg based on the
sort being reversed in a previous PR.
db507be41a