Add additional message for proxy users about https//.insteadOf git://, fixes #250.

@pixelheresy can you disable `https://.insteadOf git://` in your `git` config and test this out?
Also, if you find a good way to detect proxy ONLY errors in https://github.com/bower/bower/pull/756/files#L0R147, perhaps with a RegExp or something, it would be really cool. At the moment, that suggestion is being shown for every error.

//cc @sindresorhus