Handle rufus_scheduler v3 also

Stop rufus_scheduler v3 leaking ActiveRecord handles. See [this StackOverflow answer for details](http://stackoverflow.com/a/19380690/1270789).

rufus_scheduler v3 [deprecated `start_new`](https://github.com/jmettraux/rufus-scheduler/blob/master/lib/rufus/scheduler.rb#L120), so instead I call `new`. In [v2.0.22 this creates a `PlainScheduler`](https://github.com/jmettraux/rufus-scheduler/blob/82b70feeb28b2aa778d1d869aba12b580966a786/lib/rufus/scheduler.rb#L33), but fortunately the defaults that Huginn uses means that `start_new` in v3.0.x also creates a `PlainScheduler`.