Fix for irb crash during completion

For some Ruby implementations Readline.point is missing. This causes irb
to crash under Ruby Enterprise Edition 2011.12 for any irb completion
that is attempted. This change at least avoids the crash though the
original function (opening the text editor) cannot be executed anymore.

lib/pry-editline.rb

@@ -48,7 +48,9 @@ def self.hijack_inputrc
 
   def self.completion_proc
     lambda do |s|
-      if Readline.point == 0 && Readline.line_buffer =~ /  $/
+      if Readline.respond_to?(:point) && Readline.respond_to?(:line_buffer) &&
+        Readline.point == 0 && Readline.line_buffer =~ /  $/
+      then
         require 'tempfile'
         Tempfile.open(['readline-','.rb']) do |f|
           f.puts(Readline.line_buffer[0..-3])