Homebrew has been making a number of changes towards improved support
for installing multiple versions side by side. As part of this change,
autoconf213 has been renamed to autoconf@2.13,
and has been migrated from the `homebrew-versions` repository
(which is being deprecatetd) to the `homebrew-core repository`,
meaning `homebrew-versions` no longer needs to be tapped.
Additionally, the improved side-by-side installation support means
both versions cleanly install together, meaning the custom script
to handle linking is no longer needed.