As of R1 2017 the Kendo UI widgets switched from using image-based icons to font-based ones. That resulted in some changes in the functioning of the widgets as well as their rendering. I believe the imageClass property no longer exists for the custom command button in the Grid for that reason. To replace it there is now the className property which adds a class name to the custom command button.
To properly add a Kendo UI icon however, you either have to do heavy restyling of the button or just use a script to insert a span with the Kendo UI icon classes before the button text.
I have created a dojo example with one possible way of doing this with a script called by the dataBound event of the Grid. You can find it here: http://dojo.telerik.com/IduFUp
You can find a complete list of the built-in Kendo UI font icons and their class names here: http://docs.telerik.com/kendo-ui/styles-and-layout/icons-web