Add Python 3.6 and Django 2.0 targets to the tox file.

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

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
     coverage-clean
-    py{27,35}-django{1.10,1.11}
+    py{27,36}-django{1.10,1.11,2.0}
     coverage-report
 
 [testenv]
@@ -10,6 +10,7 @@ basepython =
     py33: python3.3
     py34: python3.4
     py35: python3.5
+    py36: python3.6
 
 commands=
     coverage run {envdir}/bin/django-admin.py test --mayan-apps --settings=mayan.settings.testing --nomigrations
@@ -20,6 +21,7 @@ deps =
     django1.9: django>=1.9,<1.10
     django1.10: django>=1.10,<1.11
     django1.11: django>=1.11,<1.12
+    django2.0: django>=2.0,<2.1 
 
 setenv=
     COVERAGE_FILE=.coverage.tox.{envname}