Register subclasses of subclasses of Jekyll::Plugin

The Sass and SCSS converters are practically the same – only different in
the input syntax and file extension. As such, we've created
`Jekyll::Converters::Scss` which is a subclass of `Jekyll::Converter`, and
`Jekyll::Converters::Sass` which is a subclass of
`Jekyll::Converters::Scss`. When `Site#instantiate_classes` is called on
`Jekyll::Converter`, it only instantiates the `Scss` converter, not the
`Sass` converter. This change fixes that.

Fixes #2334.
Update history to reflect fixing of #2334.