Revert "adapt ruby code to work with ruby 2.3"

This reverts commit eebd51a.

src/bindings/swig/ruby/kdb.rb

@@ -65,7 +65,7 @@ def pretty_print
       if meta.size > 0
         puts "  meta data keys: #{meta.size}"
         meta.each do |k|
-          puts "    " + k.to_s.sub(/^meta:\//, "")
+          puts "    " + k.to_s.delete_prefix("meta:/")
         end
       else
         puts "  key has no meta data"