Add Firefox profile with support for CSS grid to acceptance tests

The acceptance tests are currently run on Firefox 47; in that version
the CSS grid support was not enabled by default, but it could be enabled
through a setting in the Firefox profile.

By default Selenium uses a clean Firefox profile when a new session is
started, but it also allows the customization of the profile through a
zipped "user.js" file. The contents of that file have to be provided in
the "firefox_profile" capability when the Firefox session is created.

In the Mink extension for Behat several Mink sessions can be defined in
the "behat.yml" file. Each Mink session uses a different browser session
in Selenium, and each of those browser sessions is initialized with the
capabilities provided in the "behat.yml" file.

From the point of view of the acceptance tests each Mink session is an
actor, so different actors can use different browsers with different
capabilities.

Due to all this a new actor was introduced, "Rubeus", who uses a Firefox
browser that has CSS grid support; this actor is meant to be used only
in those acceptance tests that require proper support for CSS grids.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>