Use sdl dev libs

.travis.yml

     - sudo apt-get -qq -y update
 
 install:
-    - sudo apt-get -qq -y install libsdl2 libsdl2-ttf libalut-dev libpng12-dev zlib1g-dev
+    - sudo apt-get -qq -y install libsdl2-dev libsdl2-ttf-dev libalut-dev libpng12-dev zlib1g-dev
     - if [ "$CC" = "gcc" ]; then sudo apt-get install gcc-4.8; fi
     - if [ "$CXX" = "g++" ]; then sudo apt-get install g++-4.8; fi