npm: Update `main` to ref unbundled source
npm: Update `main` to ref unbundled source

Given that `main` is probably only relevant to projects that are being
built with browserify/webpack/systemjs, and in these environments,
Marionette is likely being managed individually as a dependency, it
makes sense for Backbone.Modal to specify the unbundled version as
`main`. This way, such projects can import with:

```js
require('backbone.modal');
```

rather than:

```js
require('backbone.modal/backbone.modal');
```

awkward/backbone.modal