chore(infrastructure): Don't run Chrome headless during SL runs

[acdvorak]

Also:

• Allows unit tests to run on Sauce Labs without Travis CI:
  • Assuming you have the SAUCE_USERNAME and SAUCE_ACCESS_KEY env vars set
  • Assuming tunnels to SL are permitted by your organization's security policy

[codecov-io]

Codecov Report

Merging #3210 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3210   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files         101      101           
  Lines        4444     4444           
  Branches      585      585           
=======================================
  Hits         4361     4361           
  Misses         83       83

---

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[mdc-web-bot]

🤖 Beep boop!

Screenshot test report

Commit 6089394 vs. master:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/07/25/17_58_33_707/report/report.html

No diffs! 💯🎉

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report

Commit b0c690b vs. master:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/07/25/18_18_21_395/report/report.html

No diffs! 💯🎉

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report

Commit f33a659 vs. master:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/07/25/20_15_34_876/report/report.html

No diffs! 💯🎉