fix less to stable version

3.0 is still alpha and currently breaks when building styles

.travis.yml

 before_install:
   - "export DISPLAY=:99.0"
   - "sh -e /etc/init.d/xvfb start"
-  - npm install less less-plugin-clean-css grunt-cli
+  - npm install less@2.7 less-plugin-clean-css grunt-cli
   - export PATH=`pwd`/node_modules/.bin:$PATH
   - ./manage.sh install_geckodriver ~/drivers
   - export PATH=~/drivers:$PATH