use Meteor.wrapAsync instead of Meteor._wrapAsync if defined

Hello!

Thanks for your work. I am currently using your package in a Meteor 1.0.1 project.

Everytime I set a new entry in Redis DB via this package, the Meteor server logs warns me:

```
Meteor._wrapAsync has been renamed to Meteor.wrapAsync
```

So here is a tiny pull request to use either the new name `wrapAsync` for new Meteor projects, or the former name `_wrapAsync` for older projects.