Fix deprecation warnings for Django 1.11 and Django 2.0.

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

mayan/apps/events/widgets.py

@@ -1,6 +1,6 @@
 from __future__ import unicode_literals
 
-from django.core.urlresolvers import reverse
+from django.urls import reverse
 from django.utils.safestring import mark_safe
 
 from .classes import Event