Fix for iOS 12.2

Apply fix from https://github.com/katzer/cordova-plugin-background-mode/issues/419#issuecomment-473851949

src/ios/APPBackgroundMode.m

@@ -241,7 +241,7 @@ NSString* const kAPPBackgroundEventDeactivate = @"deactivate";
  */
 + (NSString*) wkProperty
 {
-    NSString* str = @"X2Fsd2F5c1J1bnNBdEZvcmVncm91bmRQcmlvcml0eQ==";
+    NSString* str = @"YWx3YXlzUnVuc0F0Rm9yZWdyb3VuZFByaW9yaXR5";
     NSData* data  = [[NSData alloc] initWithBase64EncodedString:str options:0];
 
     return [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];