Enable yarn on travis + fix broken test

.travis.yml

@@ -3,6 +3,7 @@ node_js:
   - 6
 script:
   - npm run lint && npm run test
+  - yarn jest
   - 'if [[ ${TRAVIS_PULL_REQUEST_BRANCH:-$TRAVIS_BRANCH} = "master" ]]; then npm install -g grunt npm@5.2 && grunt pre-build; fi'
 after_success:
   - openssl aes-256-cbc -K $encrypted_440d7f9a3c38_key -iv $encrypted_440d7f9a3c38_iv