Use memoist gem instead of carrying a patch in our rails fork.

We upstreamed our patch to memoist here:
https://github.com/matthewrudy/memoist/pull/2

It has been available in memoist since:
v0.11.0 v0.10.0 v0.9.3 v0.9.2 0.9.0 0.2.0

We can now drop our rails fork patch:
https://github.com/ManageIQ/rails/commit/76ba73412f6de3abe775464bba6a8ec9434fdc98

Fixes #538