Fixed purchase event mapping

Author: Brandon Sneed

Example/Tests/Tests.m

@@ -133,7 +133,7 @@
 
         [integration track:payload];
         // TODO: look into how to handle mapping Firebase reserved params to each products
-        [verify(mockFirebase) logEventWithName:@"ecommerce_purchase" parameters:@{
+        [verify(mockFirebase) logEventWithName:@"purchase" parameters:@{
             @"checkout_id" : @"9bcf000000000000",
             @"transaction_id" : @"50314b8e",
             @"affiliation" : @"App Store",

Segment-Firebase/Classes/SEGFirebaseIntegration.m

@@ -99,7 +99,7 @@ - (NSString *)formatFirebaseEventNames:(NSString *)event
                                              kFIREventBeginCheckout, @"Checkout Started",
                                              kFIREventPresentOffer, @"Promotion Viewed",
                                              kFIREventAddPaymentInfo, @"Payment Info Entered",
-                                             kFIREventEcommercePurchase, @"Order Completed",
+                                             kFIREventPurchase, @"Order Completed",
                                              kFIREventPurchaseRefund, @"Order Refunded",
                                              kFIREventViewItemList, @"Product List Viewed",
                                              kFIREventAddToWishlist, @"Product Added to Wishlist",