Fix Travis CI builds harder

Folder names apparently change between SDL2 releases.

Author: polarina

.travis.yml

@@ -18,7 +18,7 @@ before_install:
       export PATH=/opt/ghc/$GHCVER/bin:$PATH
     fi
   - wget http://libsdl.org/release/SDL2-2.0.4.tar.gz -O - | tar xz
-  - cd SDL2-2.0.3 && ./configure && make -j && sudo make install && cd ..
+  - cd SDL2-2.0.4 && ./configure && make -j && sudo make install && cd ..
   - cabal install --only-d
 
 script: