Behat 3 upgrade

Upgrade to Behat 3

Changes:
- [x] Minor update on sentences
- [X] Update DemoContext to reflect changes of Behat Upgrade
- [x] Update ContentContext to reflect changes of Behat Upgrade

To do:
- [x] pass on that PR (https://github.com/ezsystems/ezpublish-community/pull/182)
Tbh for me it looks much more usual to use _at_ than _on_, but discussing also with @joaoinacio he also thinks the same way.
I can update these to be _on_ without problem

However should we *be strict* with sentences?
Imho no.
It's intended to be wide, giving as much as possible options to write the most readable sentences.