Rename master branch to main

This is in line with a new policy for the Servo organization discussed by the Servo TSC. We consider main to be easier to type, friendlier, and more modern so we are standardizing on this branch name for all projects in the organization.

Author: mrobinson

.github/workflows/main.yml

@@ -2,7 +2,7 @@ name: CI
 
 on:
   push:
-    branches: ['master', 'auto']
+    branches: ['main', 'auto']
   pull_request:
 
 jobs: