Check for updates once per minute when the user visits Dashboard -> Updates.

This was suggested in #8 as a way to make the custom update checker more consistent with how WP handles plugin updates. Arguably, visiting "Dashboard -> Updates" means that the user wants to check for updates, so it's okay to ignore the configured check interval in this case.

You can still disable automatic checks by setting $checkPeriod to 0, which will also disable this additional check.