Update the README links

- https links for everything
- Gitter channel has moved to Matrix
- Google groups link moved down as it is less current

README.md

@@ -14,15 +14,15 @@
 Backbone supplies structure to JavaScript-heavy applications by providing models with key-value binding and custom events, collections with a rich API of enumerable functions, views with declarative event handling, and connects it all to your existing application over a RESTful JSON interface.
 
 For Docs, License, Tests, pre-packed downloads, and everything else, really, see:
-http://backbonejs.org
+https://backbonejs.org
 
 To suggest a feature or report a bug:
 https://github.com/jashkenas/backbone/issues
 
 For questions on working with Backbone or general discussions:
-https://groups.google.com/forum/#!forum/backbonejs,
-http://stackoverflow.com/questions/tagged/backbone.js, or
-https://gitter.im/jashkenas/backbone
+https://stackoverflow.com/questions/tagged/backbone.js,
+https://matrix.to/#/#jashkenas_backbone:gitter.im or
+https://groups.google.com/g/backbonejs
 
 Backbone is an open-sourced component of DocumentCloud:
 https://github.com/documentcloud