update hostname. [ci skip]

via https://developer.github.com/changes/2014-04-25-user-content-security/

> In order to better isolate your content from potentially malicious content uploaded by other users (e.g., content that might contain Cross-Site Scripting or other embedded attacks), we now serve user-generated content from subdomains of githubusercontent.com. This content is no longer served from subdomains of github.com.

`raw.github.com` -> `raw.githubusercontent.com`