make sure to read all files in UTF-8 encoding

This fixes an encoding issue where "rake" fails because of some encoding issue:

$ rake
Building Ember Data...
rake aborted!
"\xC3" on US-ASCII

This is basically the same as https://github.com/emberjs-addons/ember-bootstrap/issues/55 / https://github.com/emberjs-addons/ember-bootstrap/issues/56