do not undef =~ unless defined (#2149)

Undefining a nonexistent definition is an error in Ruby.  This has not been a problem because there always was `Object#=~` predefined.  But we are planning to delete that useless method.  This would become an error unless properly guarded.

lib/pry/code.rb

@@ -339,7 +339,7 @@ class Pry
         super
       end
     end
-    undef =~
+    undef =~ if method_defined?(:=~)
 
     # Check whether String responds to missing methods.
     def respond_to_missing?(method_name, include_private = false)