var texture = PIXI.Texture.fromImage("https://dl.dropboxusercontent.com/s....");
With the above code, a cross domain request is created for the Sprite texture to load. This is usually not allowed (as in Dropbox case).
In order to see the sprite you will have to copy the file to the local web server or allow Cross domain requests on the other server (https://developer.mozilla.org/en-US/docs/HTTP/Access_control_CORS)
//local image instead of cross domain
var texture = PIXI.Texture.fromImage("img/player.PNG");