Expected identifier error in IE8

At line 6:

```
exports.default = thunkMiddleware;
```

`default` is a reserved keyword in IE8 and it throws an errror when dot notation is used. it does work when bracket notation is used:

```
exports['default'] = thunkMiddleware;
```

I think this is a bug in the upstream babel-plugin-add-module-exports, but wanted to show it here in case people are looking here first.
