Fix deadlinks about rubycentral

issue #587

replaced:

- http://www.rubycentral.org -> http://rubycentral.org
- http://www.rubycentral.org/rcg2006.pdf -> http://rubycentral.org/community/grant

remain deadlinks:

- http://www.rubycentral.org/conference
- http://www.rubycentral.org/conference/register.html
- http://www.rubycentral.org/grant/announce.html
- http://www.rubycentral.org/grant/application.html
- http://www.rubycentral.org/conference/prereg/
- http://www.rubycentral.org/conference/register
- http://www.rubycentral.org/soc2006
- http://www.rubycentral.org/conference/agenda.html