.travis.yml: Try to work around Travis CI

Seems their infrastructure migration went not quite as expected...

.travis.yml

 language: perl
-before_install:
-  - cpanm Shell::Command
-  - cpanm Test::Most
-  - apt-get moo
-install:
-  - sudo apt-get install cowsay git ruby-ronn
+sudo: false
+addons:
+  apt:
+    packages:
+      - cowsay
+      - git
+      - libshell-command-perl
+      - libtest-most-perl
+      - ruby-ronn
 script:
   - make test
 after_script: