Nested shim dependencies don't verify their dependencies are ready

When a module with shim dependencies depends on another module with its own shim dependencies, it doesn't verify that the nested dependencies are ready prior to executing. This will only throw an error intermittently (entirely based on timing).

```javascript
steal.config({
  shim: {
    "jquery": { exports: "jQuery" },
    "jquery-ui.js": ["jquery"],
    "jquery.multiselect.js": ["jquery", "jquery-ui.js"],
    "jquery.multiselect.filter.js": ["jquery", "jquery-ui.js", "jquery.multiselect.js"]
  }
});
```

In this case, **jquery.multiselect.filter.js** will sometimes execute prior to its dependencies being loaded as it assumes they don't have their own shim dependencies.