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.
Bugfix: Return the right blank data in _get_checks()

Otherwise we might get a traceback in
pootle_store.views.py:get_qualitycheck_stats.
Cleanups
Docs: Add note on how to disable amaGama service
Docs: Add syntax highlight for examples
Migrations: Only delete unique constraints when appropriate
Migrations: Only delete profile_id column if it exists