Update rubygems.

More vulnerabilities:
https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/4_YvCpLzL58
https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/KtmwSbEpzrU
https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/AFBKNY7VSH8