Added definition of Ruby 2.6.0 preview3
Added definition of Ruby 2.6.0 preview3

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