Update tree.md

Removed outdate „implements Node” in Gedmo\Tree documentation
Regex issue in Urlizer class

The problem is that the caret `^` is use several times in the same regex so the caret caracter isn't catch.

`preg_replace('/[^A-Z^a-z^0-9^\/]+/', '_', 'abc^def')` return `abc^def`instead of `abc_def`
The good regex is `/[^A-Za-z0-9\/]+/`
Merge pull request #848 from Abhoryo/patch-1

Regex issue in Urlizer class
Merge pull request #847 from tomaszsobczak/patch-1

Removed outdate „implements Node” in Gedmo\Tree documentation