Switch to yarn

Adds the lock file.
Removes unnecessary config and env vars.
Adds env vars so that the two tests can run in parallel.

.travis.yml

@@ -2,12 +2,14 @@ sudo: false
 language: node_js
 node_js:
   - "6"
-env: MAINRUN=true
-before_install:
-  - npm config set ca ""
-install:
-  - npm install && npm install travis-ci
+cache: yarn
+env:
+  - TEST_SUITE=coverage
+  - TEST_SUITE=browser
+after_install:
+  - yarn install travis-ci
+script:
+  - if [[ $TEST_SUITE = "coverage" ]]; then yarn run coveralls; fi
+  - if [[ $TEST_SUITE = "browser" ]]; then yarn run test-cross-browser; fi
 after_success:
-  - npm run-script coveralls && npm run-script test-cross-browser
   - if [[ $TRAVIS_BRANCH = "master" ]] && [[ $TRAVIS_PULL_REQUEST = "false" ]]; then node trigger-deploy-mn-com.js; fi
-