The memcached client wasn't correctly handling looking up a key that
didn't exist. Now we only try to push the expiration forward if there
is actually a value in memcached.
Also while I'm in here, allow expiration to be left blank.
* Update syntax to ES6
* Use `memcached` instead of `memcache`
* Fix restrictions where expirations weren't pushed forward on GET
* Fix a bug where we were unnecessarily bumping expirations on key search
Closes#201
John Crepezzi