Add define() to UMD to make proper AMD package

Hi, this may not be the best, but I had to manually do this to get backbone.iobind to play nice with AMD. Requirejs is picky about seeing `define`.

Backbone UMD pattern taken from: https://github.com/thedersen/backbone.validation/blob/master/dist/backbone-validation-amd.js

With no `define()` I encounter the error:

`Uncaught Error: Module name "backbone.<moduleName>" has not been loaded yet for context: _. Use require([])`