Update README.md; Demo link now points to a valid url.
Update README.md; Demo link now points to a valid url.

Commits to https://github.com/scottjehl/Respond/blob/master/test/test.html  automatically apply to the linked url. 
https://rawgithub.com/ is a valid host to render GitHub content with correct mime-types applied and provides information relating to hosting examples using the GitHub repository.
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