users only can have one role.

[SergioDzul]

django 1.8
when users have more than one role. the method utils.get_roles () generates an error when trying to run the filter (PrincipalRoleRelation.objects.filter) with a string. That marks the following error:

invalid literal for int() with base 10: ','

[lambdalisue]

As I said in #45, it is a issue of django-permissions. django-permission does not have any relationship between that.