travis: improve build config

Build config validation showed up 1 warning, 1 info
- deprecated sudo
- missing os

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

.travis.yml

@@ -3,8 +3,8 @@ git:
   quiet: true
 
 language: c
+os: linux
 dist: focal
-sudo: false
 cache:
   directories:
     - $HOME/sdk