Fix deprecation warnings for Django 1.11 and Django 2.0.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/django_gpg/models.py

@@ -4,8 +4,8 @@ from datetime import date
 import logging
 
 from django.core.exceptions import ValidationError
-from django.core.urlresolvers import reverse
 from django.db import models
+from django.urls import reverse
 from django.utils.encoding import python_2_unicode_compatible
 from django.utils.translation import ugettext_lazy as _