Fix libffi for rhel

I'm not totally sure this is the right way to solve this, but the commit feac1d01 (moving from shell methods to a pure Ruby implementation) broke this on RHEL.

Basically, a Ruby fileglob-way of copying versus just the shellout to "mv" don't do the same thing: `libffi.so.6` is a symlink to `libffi.so.6.0.1`, and when the latter is atomically moved to `lib`, the move of the former fails because the link is no longer found. I believe `/bin/mv` moves the whole glob as a group so this doesn't occur.