I was recently asked to add Rating refiner for the Search Results in SharePoint 2013 online site. The way we can achieve this is
Go to Search Administration à Manage Search Schema
Search for ows_AverageRating crawled property
Click on ows_AverageRating property
Map it to any of the RefinableDecimal properties
Edit the Refinement Web Part à Choose Refiners à Add the RefinableDeciaml00 property
The end result à
Hope it helps.