Switch to attributes instead of annotations

[mpdude]

Behat supports attributes since version 3.10.

Can we switch to attributes as well in MinkContext and what would that mean for users?

Given that we can raise requirements sufficiently, my guess would be that it is transparent.

[stof]

If we bump the min version of Behat, this will indeed be seamless for users.

[aik099]

According to the #48 implementation, MinkExtension would stop working on PHP 7.x. Not so seamless.

[mpdude]

We‘d need to bump the min version of Behat, effectively also bumping min PHP Version requirements