Don't install libvorbis-dev on Travis

The in-tree copy of vorbis is used. No reason to install the package anymore.

.travis.yml

@@ -9,7 +9,7 @@ script: ./travis-ci-build.sh
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get remove -qq -y mingw32
-  - sudo apt-get install -q -y libgl1-mesa-dev libsdl1.2-dev libvorbis-dev libfreetype6-dev mingw-w64
+  - sudo apt-get install -q -y libgl1-mesa-dev libsdl1.2-dev libfreetype6-dev mingw-w64
 
 notifications:
   email: false