Log Message:
Updated package to latest version, 2.8.1. Changes include:

* Features/Changes
** Compiler: reduce memory usage for shortvar
** Lib: Add optional `prefix` arg to Jsonp.call
** Ppx: add ppx_driver compatibility

* Bug fixes
** Lib: fix Js.object_keys, Js.export
** Lib: Async, do not ignore unhandled exceptions