Adding an update target to Makefile

`make update` allows to pull last version and regenerate files
Clean discourse.conf and set servers

Servers set:

* Database server ... db
* Cache server ...... cache

This allows to start the web container with the following links
to the database (PostGreSQL) and cache (redis) containers and let
Discourse know to use these containers network addresses:
--link discourse-postgres:db --link discourse-redis:cache

The configuration file has been cleaned, to ease reading.

The mini profiler is disabled.