Marked amount as abstract

ref #11

src/models/Donation.ts

@@ -31,6 +31,5 @@ export abstract class Donation {
    * The donation's amount in cents (or whatever your currency's smallest unit is.).
    * The exact implementation may differ for each type of donation.
    */
-  @IsInt()
-  amount: number;
+  abstract amount: number;
 }