Fix store.cleanSnapshots for redis implementation

[TyGuy]

Before

If you have an EventStore instance of type redis including a non-zero maxSnapshotsCount (for example, maxSnapshotsCount: 5),
When you have fewer than 5 snapshots (i.e. 0), and you call something like:

es.createSnapshot(snap, (err) => {
    if (err) { console.log(err); /*handle err*/ }
    // ...etc.
})

Then you will see this error:

ReplyError: ERR wrong number of arguments for 'del' command
/app/node_modules/redis-parser/lib/parser.js in parseError at line 193:12

(NOTE: the snapshot does get created correctly, but then the return value errors because cleanSnapshots fails).

Now

You can call es.createSnapshot(snap, cb) without the error from cleanSnapshots.

@adrai hope this is helpful, and let me know if you see any changes you'd like made. Thanks.

(Fixes #126)