Update v.2.5.1 — critical fix for stores with HPOS disabled

A critical hotfix released three days after v.2.5.0. If your store runs with High-Performance Order Storage turned off, update immediately.

• Fixed: fatal error on the order-received and view-order pages when HPOS is disabled. The page returned HTTP 500 right after payment, so the customer never saw the thank-you screen. The cause was a call to $order->get_currency() inside the very handler registered on the order currency filter — the handler asked for the value it was itself producing, and the call recursed until PHP ran out of memory. The fix uses the currency argument that the filter already passes in.
• Added the missing five-minute cron interval. Selecting "every 5 minutes" for automatic rate updates used to register an interval WordPress did not know, so the schedule silently never fired. That option now works.

The bug was invisible on any site with HPOS enabled, which is why it survived testing. Stores on HPOS were never affected.