Document that PhantomJS is required to build Channel Manager and Hippo-theme

PhantomJS is required to execute the tests for Hippo-Theme and the channel manager.

This is not documented on http://www.onehippo.org/library/development/build-hippo-cms-from-scratch.html

Another option is to automaticly install PhantomJS with the Maven PhantomJS plugin, http://kylelieber.com/phantomjs-maven-plugin/