[Linter] Cleanup the github sources checks

Code climate hates this function so I pulled it apart a little bit. Also since you can't have a gist that starts with `www.`, added a check for
that as well. Related to #48 as this is another small improvement to the linter.
`perform_github_uri_checks(git_uri) if git_uri.host.end_with?('github.com')`