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).

lib/pry/pry_class.rb

@@ -247,6 +247,15 @@ def self.default_editor_for_platform
   end
 
   def self.auto_resize!
+    ver = Readline::VERSION
+    if ver[/edit/i]
+      warn <<-EOT
+Readline version #{ver} detected - will not auto_resize! correctly.
+  For the fix, use GNU Readline instead:
+  https://github.com/guard/guard/wiki/Add-proper-Readline-support-to-Ruby-on-Mac-OS-X
+      EOT
+      return
+    end
     trap :WINCH do
       begin
         Readline.set_screen_size *Terminal.size!