ci: use travis-ci.com as .org is decomm'd

- https://blog.travis-ci.com/2021-05-07-orgshutdown

- made the switch over with my last PR, but needed to change the docs
  too
  - update badge to use `.com`
  - update link to use `.com`, `app.` subdomain, and `github/` URL
    that travis-ci.com currently uses

- also use `/branches` URL as the default URL shows current builds,
  which includes WIP PR/branch builds too
  - `/branches` shows the "default branch" first instead and various
    checkmarks etc, so it's a good summary page that can be drilled
    down from

README.md

@@ -15,7 +15,7 @@
 [![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/main.svg)](https://travis-ci.org/agilgur5/react-signature-canvas)
+[![build status](https://img.shields.io/travis/com/agilgur5/react-signature-canvas/main.svg)](https://app.travis-ci.com/github/agilgur5/react-signature-canvas/branches)
 [![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)