[ruby/irb] Drop ColorPrinter's workaround for BasicObject

(ruby/irb#1051)

`pp` 0.6.0+ includes ruby/pp#26 to handle BasicObject,
so we can drop the workaround.

ruby/irb@08908d43c7

lib/irb/color_printer.rb

@@ -4,9 +4,6 @@
 
 module IRB
   class ColorPrinter < ::PP
-    METHOD_RESPOND_TO = Object.instance_method(:respond_to?)
-    METHOD_INSPECT = Object.instance_method(:inspect)
-
     class << self
       def pp(obj, out = $>, width = screen_width)
         q = ColorPrinter.new(out, width)
@@ -28,8 +25,6 @@ def pp(obj)
       if String === obj
         # Avoid calling Ruby 2.4+ String#pretty_print that splits a string by "\n"
         text(obj.inspect)
-      elsif !METHOD_RESPOND_TO.bind(obj).call(:inspect)
-        text(METHOD_INSPECT.bind(obj).call)
       else
         super
       end

lib/irb/irb.gemspec

@@ -42,4 +42,5 @@ Gem::Specification.new do |spec|
 
   spec.add_dependency "reline", ">= 0.4.2"
   spec.add_dependency "rdoc", ">= 4.0.0"
+  spec.add_dependency "pp", ">= 0.6.0"
 end