Support Python 3.6 with Django 1.11-alpha

Only real problem is in json serialization tests:
Python 3.6 [changed][1] the json serialization
error message to use the object's class name
rather than its repr. E.g.:
  "Decimal('19.99') is not JSON serializable"
becomes:
  "Object of type 'Decimal' is not JSON serializable"

Update tests that looked for specific serialization
error message to just look for the word "Decimal"
instead. (Works with all Python versions.)

[1]: https://bugs.python.org/issue26623

.travis.yml

@@ -30,9 +30,9 @@ matrix:
     - { env: DJANGO="--pre django", python: 3.5 }
     - { env: DJANGO="--pre django", python: 3.6 }
     - { env: DJANGO="--pre django", python: pypy }
-
-  allow_failures:
-    - python: 3.6
+  # allow_failures:
+  #   - env: DJANGO="--pre django"
+  #   - python: 3.6
 
 cache:
   directories: