Add cache-service-memcached reference, update tests using memcached cache module

[sergesemashko]

Hey @jpodwys, thanks for the cache-service!
I've created cache-service-memcached. I think, it would be useful to reference it in cache-service.
Also, tests are updated using cache-service-memcached.
Please, take a look.

[jpodwys]

Thank you for this! It would be great if you wanted to reference your memcached repo from the superagent-cache and superagent-cache-plugin docs.

[sergesemashko]

thanks, I'll update docs there then too

[jpodwys]

I cut a new release so people on NPM will see your new docs update. Released as 1.3.9.