Use py3.6 in dev docker

Dockerfile

@@ -1,7 +1,7 @@
 # This dockerfile is only meant for local development of Misago
 # If you are looking for a proper docker setup for running Misago in production,
 # please use misago-docker instead
-FROM python:3.5
+FROM python:3.6
 
 ENV PYTHONUNBUFFERED 1
 ENV IN_MISAGO_DOCKER 1