Programmatically change redirect after logout
hook_user_logout() doesn't expect a redirect response object and won't respond to it. 

One alternative here, if you don't need a programmatic forcing of redirection, is to use the destination query parameter. The path supplied in the parameter will be redirected to upon logout. For example: http://example.com/user/logout?destination=user/login will return a user to the login page following logout.