[fix] install npm packages locally on .travis.yml

for some reason installing grunt globally caused travis test to fail.

.travis.yml

 before_install:
   - "export DISPLAY=:99.0"
   - "sh -e /etc/init.d/xvfb start"
-  - npm install -g less grunt-cli
+  - npm install less grunt-cli
   - ( cd searx/static/themes/oscar;npm install; cd - )
 install:
   - ./manage.sh update_dev_packages