use more supported version of export default

The old es6 module transpiler does not support
`export default function whatever` syntax. This
is what some downstream libraries like Ember use.

Essentially reverts
6bd655a72a6d72f26fc69611dbe43cac6bb8c944.

refs emberjs/ember.js#9939
Merge pull request #345 from tildeio/fixes-for-old-es6-module-compiler

[BUGFIX release] use more supported version of export default