I should have locked down the version of easel in the package.json file. At the moment the version is "*" but it should have been a specific number. For now, you can fix the problem by pointing grunt copy to the right version number.
Goto Gruntfile.js and change line 16 to 'bower_components/easeljs/lib/easeljs-0.8.0.combined.js' (or which ever version is being loaded into bower_components.
Then goto app/index.html and change line 11 to <script src="scripts/easeljs-0.8.0.combined.js"></script>