Update README to reflect reading Rails timezone

It appears that rufus-scheduler version 3.3.3 [uses the Rails timezone](jmettraux/rufus-scheduler@dc9e9df) if Rails is detected. [This commit](4b7eb81) bumps the version of rufus-scheduler, allowing it to upgrade to 3.3.x, which includes the change.

This change modifies the README regarding cron syntax, to indicate that the current version of sidekiq_scheduler does use the Rails timezone when parsing cron statements.

README.md

@@ -272,8 +272,9 @@ Sidekiq.get_schedule
 
 ## Time zones
 
-Note that if you use the cron syntax, this will be interpreted as in the server time zone
-rather than the `config.time_zone` specified in Rails.
+Note that if you use the cron syntax and are not running a Rails app, this will be interpreted in the server time zone.
+
+In a Rails app, [rufus-scheduler](https://github.com/jmettraux/rufus-scheduler) (>= 3.3.3) will use the `config.time_zone` specified in Rails.
 
 You can explicitly specify the time zone that rufus-scheduler will use: