travis-ci: explicity use container-based infrastructure

Set `sudo: false` to explicitly use the (faster) container-based
infrastructure for the Travis-CI Linux build.

More info:
https://docs.travis-ci.com/user/ci-environment/#Virtualization-environments

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.travis.yml

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