Add instruction to install phantomjs?

[raq929]

I could not get tests to run until installing phantomjs npm install -g phantomjs-prebuilt as listed in the ember docs. https://ember-cli.com/user-guide/#phantomjs

[gaand]

I think this happens during install fest.

It certainly doesn't hurt to mention is as a dependancy.

Also, let's go with npm install --global ....