Fixes #20502 -- i18n test bug discovered by Travis.

If the following tests are run in this order:

* defaultfilters.tests.DefaultFiltersTests.test_localized_filesizeformat
* i18n.tests.AppResolutionOrderI18NTests

then the second test will fail because the first has cached the 'de'
translation before the second one changes INSTALLED_APPS to install
the test app, which has its own translation.
Update ticket_19891 to recent master.

Conflicts:
	tests/i18n/tests.py
Add the latest Travis configs from aptivate/django/travisci.

This should hopefully generate something which passes Travis CI tests
and can be reviewed for merge into Django master.
/etc/init.d/xvfb is not executable on Travis VMs.
Merge branch 'ticket_19891' of ../django into travisci

Conflicts:
	.travis.yml
	tests/travis_configs/before_script.sh