Add TruffleRuby 1.0.0-rc15.
Add TruffleRuby 1.0.0-rc15.

rbenv/ruby-build,dark-panda/ruby-build,sstephenson/ruby-build,OiNutter/node-build,dark-panda/ruby-build,makandra/ruby-build,nodenv/node-build,brigade/ruby-build,rbenv/ruby-build,brigade/ruby-build,ryanfb/ruby-build