Include Ember CLI and PhantomJS as "prerequisites" to the default app blueprint's README

It may seem obvious, but given that the README is one of the first things someone new to an Ember project might see and given that it shows commands that depend on Ember CLI and on PhantomJS, it seems like both of these should be listed up-front in the "prerequisites" section.

(I didn't think this warranted updating the CHANGELOG, but I can if desired.)