Changed "main" to string

This fixes issue with Browserify https://github.com/enyo/dropzone/issues/597

According to [this](https://www.npmjs.org/doc/package.json.html), `main` is an ID, I think a string is better suited for this than an array of one item. I'm not sure, if setting this field an array is even legal.
Moreover, thanks to `main` being a string, this module can work with Browserify.