Make the `max_version` migration reversible
Make the `max_version` migration reversible

This version of the down function won't necessariliy 100% replicate the
data that was dropped, but it'll get pretty darn close and probably
close enough for our purposes if we end up actually having to roll this
back.

steveklabnik/crates.io,steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io,steveklabnik/crates.io,rust-lang/crates.io,rust-lang/crates.io