Install ancestry gem

The ancestry gem allows ActiveRecord model to be organised
as a tree structure (or hierarchy). This will be applied
to the Category model so that they can form a tree of
categories and their subcategories.

Signed-off-by: Joan Tiffany Siy <joan@teamcodeflux.com>
Remove unused helper spec.

Signed-off-by: Joan Tiffany Siy <joan@teamcodeflux.com>
Create the category model

Category has ancestry, meaning each category is a tree
node. The database table will keep track of it's parent
category as well as it's depth.

Signed-off-by: Joan Tiffany Siy <joan@teamcodeflux.com>