[boost] Boost Master branch is now closed for the beta

Except with explicit permission by a release manager. Do whatever you want on develop, just don't change master. We'll re-open master after the beta. -- Marshall Upcoming release schedule: 8-Jul: Boost 1.65.0 closed for all changes 12-Jul: Boost 1.65.0 beta; master branch re-opened 2-Aug: Release closed for all changes, except by permission of a release manager. 5-Aug: Boost 1.65.0 closed for all changes 9-Aug: Boost 1.65.0 release -- Marshall