Upgrade python to 3.6 and codify Django v2.n and Dash 1.10 restrictions (#239)

* Upgrade minimum python to 3.6

* Add missing websocket file and restrict to Dash 1.10 and Django 2.n

Co-authored-by: Mark Gibbs <mark@gibbs.consulting>

Author: delsim

.travis.yml

@@ -3,7 +3,7 @@ dist: xenial
 language: python
 
 python:
-  - "3.6"
+  - "3.7"
 
 cache:
   - pip

dev_requirements.txt

@@ -2,7 +2,7 @@ coveralls>=1.6.0
 channels>=2.0
 channels-redis
 daphne
-Django>=2.0,<2.3
+Django>=2.0
 django-bootstrap4
 django-redis
 dpd-static-support>=0.0.4