Rails 6.0.0 rc1 released

CRANK

Okay, so we didn’t quite make our aspirational release schedule, but Rails 6 is now almost ready, so here’s the first release candidate! There isn’t anything remarkably new compared to the beta releases, but everything has been polished (in about 1000 commits!), and we believe this candidate could become the final release, lest we find any egregious bugs.