Update Servo buildbot master config.
No bug: spell 'build' VLAN correctly; a=nodechange
bug 861283: initia build support for Servo - make Bors wait for Mac builds before merging to master. r=org-only

--HG--
extra : rebase_source : dcf306058e7ba3527a04a0d31a8b096c16c02318
Bug 879290: add stdlib; r=rail