Update percona-toolkit.rb

Latest percona-toolkit. PT 2.2 is largely backward-compatible with 2.1, but some tools were changed significantly (e.g. pt-query-digest, pt-upgrade, and pt-online-schema-change) http://www.mysqlperformanceblog.com/2013/03/25/mysql-5-6-compatible-percona-toolkit-2-2-released/