Switch from master to main

[gembarrett]

Master-slave is a gross metaphor and "main" makes more sense: https://www.hanselman.com/blog/EasilyRenameYourGitDefaultBranchFromMasterToMain.aspx

[gembarrett]

This was so easy to do, people don't have an excuse for not making the switch. And really, GitHub should set main as the default for all future repositories to help with this.