No need to build GLFW

.travis.yml

@@ -13,10 +13,6 @@ before_install:
   - if [ $TRAVIS_OS_NAME == linux ]; then 
       sudo apt-get update -qq;
       sudo apt-get install -y --no-install-recommends libglfw3 libglfw3-dev libglew-dev;
-      wget https://github.com/glfw/glfw/releases/download/3.3/glfw-3.3.zip;
-      unzip glfw-3.3.zip && cd glfw-3.3;
-      cmake -DBUILD_SHARED_LIBS=true -DGLFW_BUILD_EXAMPLES=false -DGLFW_BUILD_TESTS=false -DGLFW_BUILD_DOCS=false .;
-      sudo make -j $CPU_NUM install && cd ..;
     fi
 
 script: