[boost] Updated 1.60 release schedule

Since we were late on the 1.60 release, we've updated the release schedule. The master branch is currently open for bugfixes and documentation changes ONLY. The master branch will close on Sunday night (Dec 12th). We will build an RC for the 1.60 release Monday, and if all goes well, release it on Thursday, December 17th. So get those bug fixes in before Sunday! -- Marshall