Follow redirects when fetching root keys

This URL changed from raw.github.com to raw.githubusercontent.com, but
curl wasn't being told to follow redirects. This updates the URL and
adds the appropriate flag to guard against future such redirects. When
using wget, redirects are followed by default, so this already works.
Merge pull request #59 from puppetlabs/follow-redirects

Follow redirects when fetching root keys