Defuse time bomb on EditLine Readline wrapper

If you are using the basic OS X Readline impl, and you Pry.auto_resize!,
you'll find that their Readline.set_screen_size = ___ segfaults.

Instead of letting this happen, we'll tell the user about it and point
them to the fix (of using GNU Readline).
Reopen Readline.output if it is not a tty