Follow GitHub redirects for `raw` links

GitHub recently changed domains for user content, such as `raw` links,
and Net::HTTP::Get doesn't follow redirects at all by default (AFAIK),
so it's up to the 'user' to handle this.

`open-uri` on the other hand actually follows redirects, so why not use
it?
Merge pull request #99 from mthssdrbrg/follow-github-redirects

Follow GitHub redirects for `raw` links