travis: fix config validation warnings

private keys should be prefixed with an underscore (_). the sudo key is
deprecated and doesn't influence anything anymore.

.travis.yml

@@ -1,6 +1,6 @@
 language: c
 
-macbase:
+_macbase:
   - &macOld
     os: osx
     compiler: clang
@@ -43,8 +43,6 @@ dist: bionic
 services:
   - docker
 
-sudo: required
-
 env:
   global:
     # Coverity token