Automatic browser tests with SauceLabs in Travis

This patch adds saucelabs support to lodash builds! See https://travis-ci.org/pimterry/lodash/jobs/13148270#L350 for an example result.

As set up here it's automatically running 3 builds, for some chosen firefox/chrome/IE versions, but this can be extended however you'd like by changing the platforms variable in the top of saucelabs.js.

**Note that this will not pass in lodash's travis build until saucelab credentials in .travis.yml are updated**. To do this, in .travis.yml you should update the SAUCE_USERNAME env var to an active saucelabs user account, and update the password by running `travis encrypt SAUCE_ACCESS_KEY=my-secret-sauce-access-key` and copying the output to replace the `secure: "KBETrs7...` line.