Add manifest for tox tests on travis

.travis.yml

@@ -2,7 +2,7 @@ sudo: false
 language: python
 
 matrix:
-    include:
+  include:
     - python: 2.7
       env: TOXENV=py27-django19
     - python: 2.7
@@ -33,8 +33,11 @@ matrix:
     - python: 3.6
       env: TOXENV=py36-django20
 
+    - python: 3.6
+      env: TOXENV=lint
+
 install:
   - pip install tox
 
 script:
-    - tox
+  - tox