Enable environment variable: SAUCE_USERNAME and SAUCE_ACCESS_KEY

Enable environment variables: *SAUCE_USERNAME* and *SAUCE_ACCESS_KEY*

Like [SauceLabs doc](https://saucelabs.com/opensource/travis) and [grunt-saucelabs](https://github.com/axemclion/grunt-saucelabs)

Doing that, we can use [secure environment variables](http://about.travis-ci.org/docs/user/build-configuration/) in Travis CI.