The existing code forces the transaction to return a bluebird promise, which is incompatiable with es6 promises and async/await.

Thus if the user application is written in es6 promises with async/await, and have to return a bluebird promise in order to use bookshelf transactions, it becomes a burden to translate.
The bookshelf.transaction is an alias for knex.transaction(http://bookshelfjs.org/#Bookshelf-instance-transaction), and the knex.transaction uses any as a return type instead of promise.

I updated the return type of the user supplied function to PromiseLike, which both es6 and bluebird promises implement.

Tested with both bluebird and es6 promises.

types/bookshelf/index.d.ts

@@ -17,7 +17,7 @@ interface Bookshelf extends Bookshelf.Events<any> {
 	Collection: typeof Bookshelf.Collection;
 
 	plugin(name: string | string[] | Function, options?: any): Bookshelf;
-	transaction<T>(callback: (transaction: knex.Transaction) => BlueBird<T>): BlueBird<T>;
+	transaction<T>(callback: (transaction: knex.Transaction) => PromiseLike<T>): BlueBird<T>;
 }
 
 declare function Bookshelf(knex: knex): Bookshelf;