Fix incorrect loading of chai.js in grunt task

Bug introduced in Pull Request #169, 7d5871cb62c59a37f3b748a70ed510e62b57598a

The error this is solving occurs when $ grunt test is run. The output currently is:

WARN [watcher]: Pattern "#{app_dir}/test/bower_components/chai/chai.js" does not match any file.
INFO [Chrome 32.0.1700 (Mac OS X 10.9.0)]: Connected on socket 77vEg3nxwNCUOe-v8AXO
DEBUG: 'DEBUG: -------------------------------'
DEBUG: 'DEBUG: Ember : 1.3.1'
DEBUG: 'DEBUG: Ember Data : 1.0.0-beta.5'
DEBUG: 'DEBUG: Handlebars : 1.3.0'
DEBUG: 'DEBUG: jQuery : 2.0.3'
DEBUG: 'DEBUG: -------------------------------'
Chrome 32.0.1700 (Mac OS X 10.9.0) ERROR
Uncaught ReferenceError: chai is not defined
at #{app_dir}//test/support/initializer.js:6