using npm instead of apt-get in .travis.yml

.travis.yml

 before_install:
   - "export DISPLAY=:99.0"
   - "sh -e /etc/init.d/xvfb start"
-  - sudo apt-get -qq update
-  - sudo apt-get -qq install less node-less
+  - npm install -g less
 install:
   - "make"
 script: