Add -L flag to cURL command to follow any redirects

cURL by default will not follow redirects, leading
to blank file unless the `-L` flag is included since
all resources once hosted on raw.github.com are now
on the raw.githubusercontent.com domain.

Fixes issue mentioned at https://github.com/git/git-scm.com/issues/422
Merge pull request #811 from randomecho/chap-9-git-internals

Update URL used in cURL to fetch from Grit library