alerts-server: Fix a rendering bug on FixedPriceAlertsTask.

While rendering the email text to send, the current price of the
related currency was being passed as BigDecimal to the MessagesApi
which uses java.text.MessageFormat, this leads to round very small
values to 0 which is useless.

alerts-server/app/com/alexitc/coinalerts/tasks/FixedPriceAlertsTask.scala

@@ -118,7 +118,7 @@ class FixedPriceAlertsTask @Inject() (
     val message = messagesApi(
       messageKey,
       event.alert.currency.string,
-      event.currentPrice,
+      event.currentPrice.toString,
       event.alert.market.string)
 
     percentageDifferenceMaybe.map { percent =>