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)
> Well, let's google it.

So what about : `I see` and `I should see`? should we google it too?