requirements: Update Django requirements
requirements: Update various requirements versions

Also drop exact versions for travis as we will always want the latest ones.
requirements: Update Raven optional dependency to 5.x
Travis: Always use latest packages from pypi
Travis: Use travis_retry for installing things
Travis: Cache apt packages
settings: Remove the INTERCEPT_REDIRECT option from dev settings

The option is deprecated on newer versions of django-debug-toolbar
Upgrade: Fix pootle imports
Migration: initial migration script
Migration: initial 2.5.1 data for sqlite and MySQL

No data for PG at 2.5.1 as it is impossible to deploy on PG for 2.5.1.
These errors are fixed later.  Most likely we could create dumps from
older version for PG migration testing.
Migration: notes on version migration setup and operations
Travis: use different dbname for testing

Prevents any potential conflict if you run this locally
Travis: run migration tests