Profile: Get rid of unused fields
Migration: fixes postgress install

Store requires that pootle_app migration has already run and thus Directory is
available.  Other DBs seem to manage as they aren't as strict as Postgres. All
Postgres fans shout yay.
Profile: Get rid of PootleProfile.*_submission_count()
Travis: always with with --traceback
Migrations: Fix a copypaste issue in migrations
Bugfix: exclude must be a tuple

Fixes the bug:

    LanguageForm.Meta.exclude cannot be a string.
refresh_stats: Add some additional prints to report progress
refresh_stats: Marginal performance improvement on large projects
Stats: Add fields to cache last created unit and last submission

This will improve performance when retrieving the stats table.
Stats: Update last created unit

Current approach will increase load when adding whole new stores,
translation projects and projects.
Stats: Update last submission
refresh_stats: Update last unit and last submission fields
Stats: Update and retrieve cached unit and submission