Use the CDN URL for plugin downloading/checking

It seems like qs0.qsapp.com is really slow right now, making plugin downloads slow (even though plugins are downloaded from the CDN, which is fast).
The problem seems to be with the redirect from qs0.qsapp.com → cdn.qsapp.com (which uses Cloudflare)

I've updated the URLs QS uses to go directly to the CDN