Remove `sudo: false` in .travis.yml

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>

.travis.yml

@@ -14,4 +14,3 @@ before_install:
 install: make stamp-npm
 before_script: make serve_bg
 script: make check
-sudo: false