cubejs-elasticsearch-driver not compatible with v7.x

**Describe the bug**
The backend package works with elasticsearch up to v6.8. With v7.x it causes the following error:

```
ResponseError: illegal_argument_exception
at onBody (/cube/node_modules/@elastic/elasticsearch/lib/Transport.js:337:23)
at IncomingMessage.onEnd (/cube/node_modules/@elastic/elasticsearch/lib/Transport.js:264:11)
at IncomingMessage.emit (events.js:326:22)
at endReadableNT (_stream_readable.js:1241:12)
at processTicksAndRejections (internal/process/task_queues.js:84:21)

```

I've tested this behaviour both with elastic.io and a local vanilla elasticsearch v7.15 image.
On the other hand, the library works correctly with elasticsearch v.6.8.18

**To Reproduce**
Steps to reproduce the behavior:
- Deploy elasticsearch v7.15 using docker.
- Try to connect cube.js by following the instructions on the wiki
- Open the cube.js schema and see the error.

**Expected behavior**
The lib should work with the latest version of elasticsearch