You can use the database cache backend, or even the FS cache backend. By "you can", I mean that "it will work". Just configure it normally for your django project and sorl-thumbnail will use it.
As per sorl-thumbnail documentation:
a cached database which requires no special installation to your normal Django setup besides installing a proper cache like memcached
But it won't be as awesome as memcached or redis.
(Also: why does sorl-thumbnail require this now ? It used to just check the FS to ensure that the thumbnail exists or else create it)