Removed the Urlizer class and reuse behat/transliterator

Both classes are exactly the same. Behat and the Gedmo extension have both copied the Doctrine 1 code in their own project. For Behat 3, the code has been moved to a package on its own to allow reusability with composer. So we can now depend on it instead of maintaining a separate copy of the same code.

FYI, we contacted the Doctrine team before putting the separate package in the behat organization, but they were not interested into providing this Doctrine1 code on its own given that Doctrine 2 does not use it anymore.

This change has absolutely no impact on the Sluggable extensions:
- the urilizer data are exactly the same in both projects
- the ``Gedmo\Sluggable\Util\Urlizer`` class and the ``Behat\Transliterator\Transliterator`` class are the same except for code formatting and for the fact that Transliterator is abstract (it only contains static methods anyway)
- the ``Gedmo\Sluggable\Util\Urlizer`` is kept in the Gedmo extension, extending the Transliterator class, thus ensuring 100% backward compatibility.