Previously we constrained the CI install plans to use only patched
versions of any packages for which we had patches. This was an attempt
to decouple ourselves from the evolution of Hackage a bit. Specifically,
I was worried that if we didn't constrain the install plan to the
patched versions then we would end up having spurious build failures
every time a new minor package version is released (since
cabal-install would prefer the new release over our patched
version). Admittedly this is just a hack to work around the fact
that version bounds are generally quite loose, but it seemed like a
reasonable trade-off at the time.

However, this ended up making it hard to support older major versions
of a package simultaneously with the newest version if the latter
doesn't require a patch. Let's try doing away with the constraints.