Background not appearing

Okay, so I told a friend that I’d re-render an animation of his on my much faster computer, but when I render it the background doesn’t show up, instead the sky is just a solid, dark purple color. I tried just adding a giant sphere and adding the texture to that, but it still renders it as a purple sky. We had this working correctly just a few days ago, so I’m not sure what happened to break it, and I’m sick of banging my head against it. I uploaded the .blend and texture to:

https://drive.google.com/file/d/0By2Mfv6zO9SkYnl0aGIxZmM0QVU/edit?usp=sharing

I’d greatly appreciate it if anyone could take a look and tell me why it’s not working right. This is using cycles, and I’m on Blender 2.71.

When I render, I got part purple rectangle and part astro image.

I see an object named Plane with a bright purple texture. It is hidden - not visible in 3D view but in the render. Use Alt-H to make it visible.

Maybe that was a test object or some early scaffolding you forgot about? Whatever, the problem is easily fixed.

Sounds like an image texture is missing. Get your friend to pack all textures, in external data in the file menu, then save his blend file. Try that, it should render then.