Update the Prerequisite section of product documentation

[kcullimore]

The installation procedure in the product document are referencing older npm & ruby versions. Are these still accurate?

I've seen older macOS computers have issues installing the current version of bundler with older versions of ruby (~2.1). Also, if an older copy of ruby is not removed and a newer version is installed in addition the older path is kept by default leading to further confusion.

Perhaps some instruction of how to handle updating ruby properly or using a rvm based ruby might help solve issues when older installations exist on a macOS machine.