Add Sauce Labs matrix to the README

This also updates the Travis CI image to be a vector.

README.md

@@ -31,6 +31,10 @@ Browser compatibility
 * Safari 3+
 * Opera 10.6+
 
+Select2 is automatically tested on the following browsers.
+
+[![Sauce Labs Test Status][saucelabs-matrix]][saucelabs-status]
+
 Usage
 -----
 You can source Select2 directly from a CDN like [JSDliver][jsdelivr] or
@@ -91,7 +95,9 @@ The license is available within the repository in the [LICENSE][license] file.
 [jsdelivr]: http://www.jsdelivr.com/#!select2
 [license]: LICENSE.md
 [releases]: https://github.com/select2/select2/releases
-[travis-ci-image]: https://travis-ci.org/select2/select2.svg?branch=master
+[saucelabs-matrix]: https://saucelabs.com/browser-matrix/select2.svg
+[saucelabs-status]: https://saucelabs.com/u/select2
+[travis-ci-image]: https://img.shields.io/travis/select2/select2/master.svg
 [travis-ci-status]: https://travis-ci.org/select2/select2
 
 [bootstrap3]: https://getbootstrap.com/