[it] Update URL used in cURL to fetch from Grit library

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
See also PR #811