env: switch to main branch as default

- now that GH officially supports branch renames, moving all my repos
  to use `main`
  - GH also creates redirects now, which seem to be working well for
    the badges right now, but better to move all of them anyway
  - also had to switch Codecov default branch etc

README.md

@@ -8,15 +8,15 @@
 <!-- releases / versioning -->
 [![package-json](https://img.shields.io/github/package-json/v/agilgur5/react-signature-canvas.svg)](https://npmjs.org/package/react-signature-canvas)
 [![releases](https://img.shields.io/github/tag-pre/agilgur5/react-signature-canvas.svg)](https://github.com/agilgur5/react-signature-canvas/releases)
-[![commits](https://img.shields.io/github/commits-since/agilgur5/react-signature-canvas/v1.0.5.svg)](https://github.com/agilgur5/react-signature-canvas/commits/master)
+[![commits](https://img.shields.io/github/commits-since/agilgur5/react-signature-canvas/v1.0.5.svg)](https://github.com/agilgur5/react-signature-canvas/commits/main)
 <br><!-- downloads -->
 [![dt](https://img.shields.io/npm/dt/react-signature-canvas.svg)](https://npmjs.org/package/react-signature-canvas)
 [![dy](https://img.shields.io/npm/dy/react-signature-canvas.svg)](https://npmjs.org/package/react-signature-canvas)
 [![dm](https://img.shields.io/npm/dm/react-signature-canvas.svg)](https://npmjs.org/package/react-signature-canvas)
 [![dw](https://img.shields.io/npm/dw/react-signature-canvas.svg)](https://npmjs.org/package/react-signature-canvas)
 <br><!-- status / activity -->
-[![build status](https://img.shields.io/travis/agilgur5/react-signature-canvas/master.svg)](https://travis-ci.org/agilgur5/react-signature-canvas)
-[![code coverage](https://img.shields.io/codecov/c/gh/agilgur5/react-signature-canvas/master.svg)](https://codecov.io/gh/agilgur5/react-signature-canvas)
+[![build status](https://img.shields.io/travis/agilgur5/react-signature-canvas/main.svg)](https://travis-ci.org/agilgur5/react-signature-canvas)
+[![code coverage](https://img.shields.io/codecov/c/gh/agilgur5/react-signature-canvas/main.svg)](https://codecov.io/gh/agilgur5/react-signature-canvas)
 <br>
 [![NPM](https://nodei.co/npm/react-signature-canvas.png?downloads=true&downloadRank=true&stars=true)](https://npmjs.org/package/react-signature-canvas)
 <br>