Add UMD wrapper, remove separate AMD version

Related PRs in [Backbone.Marionette](https://github.com/marionettejs/backbone.marionette/pull/1189) and [Backbone.BabySitter](https://github.com/marionettejs/backbone.babysitter/pull/39).

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**Changes:**
- Remove AMD version
- Add UMD wrapper
  - update Gruntfile
  - update component.json
  - update package.json

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**UMD Details**

- AMD
  - `exports = Wreqr`
  - `Backbone.Wreqr` is defined
- CommonJS
  - `exports = Wreqr`
  - `Backbone.Wreqr` is defined
- Browser
  - `Backbone.Wreqr` is defined

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Let me know if this pull request needs anything else. Thanks!