Add testing for Django master branch

Will help ensure that django-debug-toolbar stays compatible as Django is
developed.

Author: jdufresne

.travis.yml

@@ -2,6 +2,7 @@ language: python
 sudo: false
 cache: pip
 matrix:
+  fast_finish: true
   include:
     - python: 2.7
       env: TOXENV=py27-dj111
@@ -17,9 +18,19 @@ matrix:
       env: TOXENV=py35-dj20
     - python: 3.6
       env: TOXENV=py36-dj20
+    - python: 3.5
+      env: TOXENV=py35-djmaster
+    - python: 3.6
+      env: TOXENV=py36-djmaster
     - env: TOXENV=flake8
     - env: TOXENV=isort
     - env: TOXENV=readme
+  allow_failures:
+    - python: 3.5
+      env: TOXENV=py35-djmaster
+    - python: 3.6
+      env: TOXENV=py36-djmaster
+
 install:
   - pip install tox codecov
 script:

tox.ini

@@ -2,6 +2,7 @@
 envlist =
     py{27,34,35,36}-dj111
     py{34,35,36}-dj20
+    py{35,36}-djmaster
     flake8,
     isort,
     readme
@@ -10,6 +11,7 @@ envlist =
 deps =
     dj111: Django>=1.11,<2.0
     dj20: Django>=2.0,<2.1
+    djmaster: https://github.com/django/django/archive/master.tar.gz
     coverage
     django_jinja
     html5lib