try to cache the build dependencies

.travis.yml

@@ -7,6 +7,7 @@ language: c
 # Make sure build dependencies are installed.
 install:
  - sudo apt-get install mono-gmcs cli-common-dev libsdl1.2debian libgl1-mesa-glx libopenal1 libfreetype6 dpkg rpm nsis markdown
+cache: apt
 
 # Run the build script
 # call RALint to check for YAML errors