Rename master branch to main

[skmcgrail]

• Updates Travis jobs to refer to main as the target branch name.
• Updates documentation links to refer to main.

[skotambkar]

Thanks, additionally README - Build Status and Apache V2 License tag would need an update for master -> main.

[skmcgrail]

Build status link is fine, but will need to update the license tag, good call out, not sure how that got missed in my search :D