Fix deprecation warnings for Django 1.11 and Django 2.0.

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

mayan/apps/documents/widgets.py

@@ -1,7 +1,7 @@
 from __future__ import unicode_literals
 
 from django import forms
-from django.core.urlresolvers import reverse
+from django.urls import reverse
 from django.utils.encoding import force_text
 from django.utils.html import strip_tags
 from django.utils.http import urlencode