Prevent name collision with other gems: allowing the use of `has_closure_tree`method besides `acts_as_tree`

Scenario: one of your dependencies uses other gem like Ancestry, while you want to use this gem for its multiple benefits. Even if that dependency uses the `has_ancestry` method instead of the `acts_as_tree` method, it will overwrite ClosureTree's acts_as_tree, render it useless. If we can still use another method like `has_closure_tree`, we can still work with this.

This way, the user can choose between using `has_closure_tree` (more intention revealing name in terms of what tool we want to use, IMO) or the generic `acts_as_tree` as before.

I changed one of the models of the specs to use the `has_closure_tree` method while leaving the rest using the `acts_as_tree` method to ensure that everything keeps working as expected.