Add MessageContext

[wheatpenny]

In walking through the steps on the Blackbox Driver page, I realized that MessageContext needed to be added to the behat.yml file in order for the "Message selectors" section to function. I added the line to the snippet and updated the line references in the documentation page.

[jhedstrom]

Thanks! This needs to be against master however, and can be backported to the latest 3.x version.