ruby-build 20121020

• Add Ruby 1.9.3-p286
• Add JRuby 1.7.0-rc2
• Add JRuby 1.7.0-rc1
• Add JRuby 1.6.8
• Add JRuby 1.5.6
• Fetch Ruby 2.0.0.dev via Subversion instead of Git
• Allow hooks to be defined for rbenv-install