Don't assume Readline is loaded in auto_resize

lib/pry/pry_class.rb

@@ -233,15 +233,22 @@ class Pry
   end
 
   def self.auto_resize!
-    ver = Readline::VERSION
-    if ver[/edit/i]
+    Pry.config.input # by default, load Readline
+
+    if !defined?(Readline) || Pry.config.input != Readline
+      warn "Sorry, you must be using Readline for Pry.auto_resize! to work."
+      return
+    end
+
+    if Readline::VERSION =~ /edit/i
       warn <<-EOT
-Readline version #{ver} detected - will not auto_resize! correctly.
+Readline version #{Readline::VERSION} 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!)