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)
From a responsibility perspective, i'd say no... but since we supposedly depend on  what the DemoBundle wizard installs, I guess we kinda have to... for now at least. Right @MLoureiro ?