Add test that image fallbacks are correctly sized

On a scale from 1 to “*what did you just do?*”, how ridiculous is this?

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We had a bug last week that the image fallbacks we had been serving were unstyled because of PhantomJS's inability to connect to our assets host using SSLv3.

This bug was present in production for almost a month and nobody noticed because we have no automated testing of our images.

This commit introduces a very rough check for our image fallbacks. It asserts that a particular image is approximately a certain size. I expect this would only catch the most obvious breakages, but it's better than nothing.
Good question. I think it will raise a `ChunkyPNG::SignatureMismatch` (because the `@response` will be something like `<html>error</html>`), which will result in this test failing.