travis: do not use sudo, but the apt plugin

.travis.yml

@@ -1,13 +1,26 @@
 ---
 dist: bionic
-install:
+addons:
-  - sudo apt-get update
+  apt:
-  - sudo apt-get --no-install-recommends install devscripts python python3 ruby php-cli gawk ksh zsh pylint shellcheck
+    packages:
-  - sudo apt-get --no-install-recommends install pkg-config libdb-dev libvirt-dev libexpat-dev
+    - devscripts
-  # - Munin/Plugin.pm is in "munin-node" on precise
+    - python
-  - sudo apt-get --no-install-recommends install munin-node
+    - python3
-  - sudo apt-get --no-install-recommends install libsys-virt-perl
+    - ruby
+    - php-cli
+    - gawk
+    - ksh
+    - zsh
+    - pylint
+    - shellcheck
+    - pkg-config
+    - libdb-dev
+    - libvirt-dev
+    - libexpat-dev
+    - munin-node
+    - libsys-virt-perl
 
+install:
   # we use locally installed cpan modules instead of travis' builtin 'homebrew' perl support
   # The homebrew variation does not work for us, since:
   #   * it is built without threading