ThenableReference should extend Promise, not PromiseLike (#498)

ThenableReference implements a `.catch()` method, but `ref.push(...).catch(...)` generates a TypeScript error, since neither the `Reference` nor `PromiseLike` interfaces have that method.

This change was [already done on the JS SDK](firebase/firebase-js-sdk#1462) back in January, and this PR simply puts the admin SDK back in line with it.

Author: jsayol

src/index.d.ts

@@ -443,7 +443,7 @@ declare namespace admin.database {
     update(values: Object, onComplete?: (a: Error|null) => any): Promise<void>;
   }
 
-  interface ThenableReference extends admin.database.Reference, PromiseLike<any> {}
+  interface ThenableReference extends admin.database.Reference, Promise<admin.database.Reference> {}
 
   function enableLogging(logger?: boolean|((message: string) => any), persistent?: boolean): any;
 }