use common saucelabs env variable names

In most of the docs on the Saucelabs site (https://docs.saucelabs.com/tutorials/js-unit-testing/) they reference the env vars as such:

SAUCE_USERNAME
SAUCE_ACCESS_KEY
fallback for old saucelabs env var names
Merge pull request #44 from staygrimm/patch-1

use common saucelabs env variable names