update_plugins admin check

The current_user_can('update_plugins') (onesignal-admin.php line 74) fails if define(DISALLOW_FILE_MODS', true); is set in wp-config.php thus OneSignal is not available for configuration.

See this link why: https://core.trac.wordpress.org/browser/tags/4.5.3/src/wp-includes/capabilities.php#L320 how update_plugins is denied with do_not_allow.

To reproduce: add define(DISALLOW_FILE_MODS', true); to wp-config.php and try to access the OneSignal Dashboard page as an admin.

To fix: use a different capability check, like activate_plugins or manage_options, which do not have this corner case.

https://wordpress.org/plugins/onesignal-free-web-push-notifications/