Upgrade RateLimiterRedis to use node-redis v4 as storage client
More or less two months ago, a new version of [node-redis](https://www.npmjs.com/package/redis) was released. This makes version `4.x.x` the current stable version. For compatibility reasons, the [RateLimiterRedis](https://github.com/animir/node-rate-limiter-flexible/wiki/Redis) implementation should be updated to the latest version. The new [node-redis](https://www.npmjs.com/package/redis) version works with promises by default and makes promisify superfluous.

The new version of [node-redis](https://www.npmjs.com/package/redis) also brought changes to the API. Hence the question, is backwards compatibility required for version `3.x.x` or could the code be easily adapted for [node-redis](https://www.npmjs.com/package/redis) `4.x.x` in the next [node-rate-limiter-flexible](https://github.com/animir/node-rate-limiter-flexible) version? Anyway, the update should be completed, since many projects that already rely on redis for caching (e.g. database query caching) will use [node-redis](https://www.npmjs.com/package/redis) `4.x.x` in the future. Otherwise, these projects would be forced to use [node-redis](https://www.npmjs.com/package/redis) `3.x.x` because of the rate limiting. Any opinions on this?