Fix CI builds on old Ubuntu without Opus package, OSX works as expected.

Author: BorisCarvajal

.travis.yml

@@ -13,7 +13,7 @@ before_install:
          export DISPLAY=:99.0;
          sh -e /etc/init.d/xvfb start;
          sudo apt-get update;
-         sudo apt-get install -y libopusfile-dev libvorbis-dev libtheora-dev libphysfs-dev libdumb1-dev libflac-dev libpulse-dev libgtk2.0-dev pandoc;
+         sudo apt-get install -y libvorbis-dev libtheora-dev libphysfs-dev libdumb1-dev libflac-dev libpulse-dev libgtk2.0-dev pandoc;
       elif [ `uname` = "Darwin" ]; then
          brew install cmake opusfile libvorbis freetype flac physfs;
       fi