Test addon key for package configuration.

.travis.yml

@@ -1,5 +1,9 @@
 language: cpp
 
-install: libsdl1.2-dev python-ply
+addons:
+  apt:
+    packages:
+    - libsdl1.2-dev
+    - python-ply
 
 script: cd build/linux/ && make all