Issue title: ioredis: TypeError [ERR_INVALID_ARG_TYPE]: The "superCtor" argument must be of type Function
 Issue body: When trying to use IORedis with connect-redis I run into the following error:

```
The "superCtor" argument must be of type Function. Received type undefined TypeError [ERR_INVALID_ARG_TYPE]: The "superCtor" argument must be of type Function. Received type undefined
    at Object.inherits (util.js:280:11)
    at new module.exports (/Users/ajmas/Development/myproject/node_modules/connect-redis/lib/connect-redis.js:139:8)
    at Object.<anonymous> (/Users/ajmas/Development/myproject/src/config/passport.ts:25:24)
    ...
```

The code in question:

```typescript
const client = new Redis()
sessionStore = new RedisStore({
    client
});
```
Environment:
  - typescript: 3.5.1
  - connect-redis: 3.4.1
  - nodejs: 10.15.3
  - ioredis: 4.9.5
  - macOS 10.14.5
 Comments: 
 Comment 0: Turns out to be an error on my part, as I ommitted 'session' in the 'pre-constructor'. My code is now something as follows:

```typescript
import session from 'express-session';
import redisStoreProto from 'connect-redis';

const RedisStore = redisStoreProto(session);
sessionStore = new RedisStore({});
```

Open to better naming.
{'source': 'GithubIssues', 'repo_name_with_owner': 'tj/connect-redis', 'issue_id': 463171310, 'repo_stars': 2165.0}