fix(infrastructure): Remove deprecated JWT addon in .travis.yml (#2521)

Fixes #2151

The `SAUCE_USERNAME` and `SAUCE_ACCESS_KEY` environment variables are now stored in our [Travis CI project settings](https://travis-ci.org/material-components/material-components-web/settings).

.travis.yml

@@ -15,11 +15,8 @@ matrix:
     - node_js: 8
       env:
         - TEST_SUITE=unit
-        - SAUCE_USERNAME=material-sauce
       addons:
         sauce_connect: true
-        jwt:
-          secure: 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
       script: npm run test:unit && npm run posttest
       after_success:
         - codecov