Promises not removing the document in migration

## I'm submitting a...

- [x] Bug report
- [ ] Feature request
- [ ] Question

## Current behaviour
```
exports.down = function(db) {
return db.dropTable("pets");
};

// Returns:
[INFO] Defaulting to running 1 down migration.
collection migrations
collection pets

```
The code above remove the collection `pets` but doesn't remove the document in the migration collection.

```
exports.down = function(db, callback) {
db.dropTable("pets1", callback);
};

// Returns:
[INFO] Defaulting to running 1 down migration.
collection migrations
collection pets1
[INFO] Processed migration 20190704145216-yop2
[INFO] Done
```
The code above works as expected, it removes the collection `pets1` and remove the right document in the migration collection.

## Expected behaviour
It should remove the right document in the migration collection when it succeed.

## Environment

db-migrate@0.11.6

db-migrate-mongodb@1.5.0

Node version: v12.4.0

Platform:   Mac

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Start by reproducing the promise-returning exports.down migration from the issue, then compare its completion path with the callback-based exports.down example. Trace the MongoDB migration handling until the migration document is removed; done means the pets collection and its corresponding migration document are both removed.

Written by the indexing model from the issue text.