https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration

`sudo: false` is getting deprecated and support for it will soon be
removed entirely. They seem to be moving to using VMs entirely for their
service.

Converse doesn't seem to be using Travis' features extensively so I
don't think anything would break, but this clause will become useless in
less than a month, (quite a tight timeline).
Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

This reverts commit 7af73c34.

Doesn't look like the right approach for adding support for XEP-0156.
Work on that will continue in a branch in the meantime.

Causes "Mixed Content" TLS errors

Currently translated at 83.1% (294 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/pl/

Currently translated at 85.9% (304 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/de/

Currently translated at 96.6% (342 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/nl/

Currently translated at 53.7% (190 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/ru/

Currently translated at 100.0% (354 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/ja/

Currently translated at 100.0% (354 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/gl/

Currently translated at 100.0% (354 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/fr/

Currently translated at 99.7% (353 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/cs/

Currently translated at 100.0% (354 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/eu/

Currently translated at 85.8% (304 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/ar/

Currently translated at 100.0% (201 of 201 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/af/

Currently translated at 24.2% (86 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/ro/

Currently translated at 100.0% (354 of 354 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/hu/

Add CSS classes to differentiate between incoming and outgoing messages

This is mainly a testing tool, which makes a mode where all HTML bugs
which could slightly mess up the rendering will trigger proper errors in
the browser.

Fixes #1131.