Fixing deprecations

[oleg-andreyev]

Changes that are required for minkphp/MinkBrowserKitDriver#138

[aik099]

I'm not sure how symfony/symfony#33236 works, but it on Travis CI somehow (maybe based on PHP version) is patching PHP files to that they'll have correct return types for overridden methods.

I like the idea, because this way we don't need to create IFs by PHP versions and different PHP classes for same thing with different method declarations.

@stof , if you like the patching idea in general, then we can start with simple sed command without actually:

1. parsing PHP file into tokens
2. adding tokens in the middle
3. saving resulted file

This way if declaration is changed in the future the build would immediately fail with same deprecation notice (since sed failed to make a replace). Then (if by that time we'll remember that sed is saving the day) we'll need to fix expression used in it.

[stof]

I suggest discussing minkphp/Mink#787 before moving forward, as this will impact the way we solve things.

[stof]

Closing in favor of #54 which is a lot simpler thanks to the ability to support only PHP 7.2+