Feat test remote testing on saucelabs

# Preview of the sauce lab result [![Sauce Test Status][saucelabs-image]][saucelabs-url]

Require #274 

[![Sauce Test Status][saucelabs-browser-matrix-image]][saucelabs-url]

Hum... The WebWorker test looks broken... I'm investigating...
https://travis-ci.org/douglasduteil/es6-module-loader#L514

[saucelabs-url]: https://saucelabs.com/u/dd-es6-module-loader
[saucelabs-image]: https://saucelabs.com/buildstatus/dd-es6-module-loader
[saucelabs-browser-matrix-image]: https://saucelabs.com/browser-matrix/dd-es6-module-loader.svg