The build name has been changed from @ckeditor/ckeditor5-build-balloon-toolbar to @ckeditor/ckeditor5-build-balloon. See ckeditor/ckeditor5#546 for more information.
The build now defines the editor as its default export. This makes requiring the build easier when using AMD, CJS or ES6 modules. See ckeditor/ckeditor5#545. (e5c7511)