[require] Check the body argument to see if it's a function before trying to call .apply on it in a multi-dependency require statement

This addresses an issue where if you require two modules simply for their side effects and you don't need to pass a callback you get an error, whereas if you make two separate require calls for them, with no callback each time, it works fine.

Calling ```_.isFunction(body)``` before trying to call ```body.apply(...)``` should do the trick. Passing a no-op callback works as well, but this seems nicer.

##### Example...

```javascript
// This works
if (Meteor.isClient){
  require('router');
  require('titleSetter');
}

// This errors
if (Meteor.isClient){
  require(['router', 'titleSetter']);
}
```

##### Here's a screenshot of the error...

![screen shot 2013-05-11 at 4 32 49 pm](https://f.cloud.github.com/assets/427516/492275/450169aa-ba93-11e2-937c-6360c926cb5c.png)

Let me know if this seems reasonable. Thanks!

Alex