Marry Date::Performance and Spreadsheet gems

Date::Performance Show archive.org snapshot is a gem that replaces various method in Ruby's Date class with fast C implementations. Unfortunately it doesn't fully implement an internal method (Date.ajd_to_jd) which makes your code blow up when you use it together with the Spreadsheet Show archive.org snapshot gem.

A solution is to restore the Ruby implementation of this particular method. To do this, copy the attached file to lib/fix_date_performance.rb to config/initializers.