Security upgrades for rack and json

Upgraded rack and json gems to address security vulnerabilities discussed in these posts:

* https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/4_YvCpLzL58
* https://groups.google.com/forum/#!msg/rack-devel/mZsuRonD7G8/DpZIOmMLbOgJ