Python3.5 is still the default Python3 version in
Debian Stretch (stable) and Ubuntu 16.04 LTS
https://packages.debian.org/source/stretch/python3-defaults
https://packages.ubuntu.com/source/xenial/python/python3-defaults

- npm install less@2.7 less-plugin-clean-css grunt-cli is moved inside manage.sh
- if it is possible to source manage.sh to update the PATH (to start grunt)
- tidy the help documentation
- update the geckodriver version to 0.19.1

3.0 is still alpha and currently breaks when building styles

WARNING: dependency changes

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

This patch should speedup testing with Travis CI using caching and
container-based infrastructure.

source:
- http://docs.travis-ci.com/user/migrating-from-legacy/
- http://docs.travis-ci.com/user/caching/

to generate the .css file from the .less file run: $make styles