Travis config update

* Dropping mirror related pip env variables
* Dropping test of unsupported Django 1.3
* Adding Django 1.7 RC1 to test environments
change default redis db from 1 to 0

This will prevent redis-py from sending SELECT calls on each connection
and make the library compatible, by default, with twemproxy.
Compatible with Django 1.7

`django.core.cache.parse_backend_uri` was dropped in [1],
so the code is now included.

[1]: https://github.com/django/django/commit/4e9f800742c3048402a
Now should pass all the travis tests

+ Removing Django 1.3 related stuff from documentation
Merge pull request #71 from remohammadi/master

Reactivating travis and updating it for 1.7