backport truncate until we've upgraded.

truncate was added in upstream in
9a4e183fe9f81365f7815c391512b3d46622d8c8
move AR::Base#truncate to manageiq repo

I'm not sure if this can be upstreamed, but it doesn't impede us from
upgrading, so we should remove it from our fork.
don't establish a connection for feature detection

we should avoid bringing up a connection too early in the boot process,
so just check the pg connection class directly
Bump the Rails fork tag to the one with the truncate method reverted.
Merge pull request #721 from tenderlove/truncate

backport truncate until we've upgraded.