Centos and ruby 1.8.7

I have foreman 1.6.0 installed and have default ruby 1.8.7 installed.

I hv an urgent demo tomorrow which needs a nokogiri gem installed but I am
unable to get through.

Can someone please guide how can I upgrade ruby on the server or is there
any other workaround ?

I have already tried install nokogiri-1.5.0 but no avail.

Please help asap.