Using VueAsyncData.mixin throws "[vue-async-data] not installed!" warning.

In vue 1.0.21 there is still mechanism, that throws "[vue-async-data] not installed!" warning 
while using vue-async-data as mixin.

```
<script>
import VueAsyncData from 'vue-async-data';

export default {
  mixins: [
    VueAsyncData.mixin,
  ],
...
  asyncData(resolve, reject) {
    CustomDataService.getItems().then(...
```

but it works properly!
