try to fix "td::exception: OGRE EXCEPTION(5:ItemIdentityException): Can not find codec for 'png' image format."

CIBuild/linux/dependencies.sh

@@ -8,5 +8,6 @@ sudo apt-get install libzzip-dev
 sudo apt-get install libois-dev
 sudo apt-get install libglu-dev
 sudo apt-get install libqt4-dev
+sudo apt-get install libpng
 wget https://github.com/paulsapps/Ogre1.9/releases/download/1.9.0/ogre-build_1.9.0-1_amd64.deb
 sudo dpkg -i ogre-build_1.9.0-1_amd64.deb