www/tdiary: Fix runtime error after www/rubygem-rackup is updated to 2.2.0

After www/rubygem-rackup is updated to 2.2.0 runtime error happens as
following.

uninitialized constant Rackup::Handler::CGI (NameError)
/usr/local/www/tdiary/index.rb:35:in `<main>'

So fix it by replacing runtime dependency to www/rubygem-rackup with
www/rubygem-rackup21.