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.
Merge pull request #231 from samkatakouzinos/patch-1

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