hotfix: from yarn to npm

Author: lauthieb

.travis.yml

@@ -26,11 +26,11 @@ before_install:
 #- export DISPLAY=':99.0'
 #- Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
 - npm install -g xvfb-maybe
-install: yarn
+install: npm install
 before_script:
 #- xvfb-maybe node_modules/.bin/karma start test/unit/karma.conf.js
-#- yarn run pack && xvfb-maybe node_modules/.bin/mocha test/e2e
-script: yarn run build
+#- npm run pack && xvfb-maybe node_modules/.bin/mocha test/e2e
+script: npm run build
 branches:
   only:
   - master