Remove memcaching require in memcache cache provider

We don't actually need to require the `memcaching` node module. Instead,
we can just check that we have a memcache client with methods we expect,
and throw otherwise. This will allow others to use other memcache clients
that are compatible with `memcaching`.
Fix lint error
Merge pull request #9 from Asana/adjust-memcaching-check

Removing memcaching require in memcache cache provider