Updated downloads page and linux-install docs

Updated downloads page and linux-install docs to use the keyserver and to explicitly note that it's not Debian-only.
As discussed with @directhex on Twitter just now, I would argue that it's not clear - I'm an experienced Linux user and went "OK, that's the Wheezy link", went ahead and dropped the appropriate ubuntu/trusty bits into the line, and it blew up.

When you're not acting as a user would expect, it strikes me that you should make that very explicit.