travis-ci: no longer use containers

Travis CI will soon deprecate the container-based infrastructure
enabled by `sudo: false` in ce59dff.

More info:
https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures

Signed-off-by: Sebastian Staudt <koraktor@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: koraktor

.travis.yml

@@ -1,7 +1,5 @@
 language: c
 
-sudo: false
-
 cache:
   directories:
     - $HOME/travis-cache