Rename "master" to "main"

[dcuccia]

Per here, probably a good thing to rename the branch master to main at some point. There's a nice helper Gist here.

[lmalenfant]

I actually did this in some of my projects and with all new projects, I definitely follow this guideline. However, I did get burned because if there are external factors involved, like pipelines, they might break by changing the name.

Possibly not an issue for us but I need to do a little more research before we make the switch.