Error querying Elasticsearch: Unknown key for a START_OBJECT in [filter]

Using `docker.elastic.co/elasticsearch/elasticsearch:5.5.3`, I see this error in `$es_response`:

```
'error' =>
array (
'root_cause' =>
array (
0 =>
array (
'type' => 'parsing_exception',
'reason' => 'Unknown key for a START_OBJECT in [filter].',
'line' => 1,
'col' => 11,
),
),
'type' => 'parsing_exception',
'reason' => 'Unknown key for a START_OBJECT in [filter].',
'line' => 1,
'col' => 11,
),
'status' => 400,
)
```

According to a quick Google search, the DSL changed in ES 5.1: https://stackoverflow.com/questions/40996266/elasticsearch-5-unknown-key-for-a-start-object-in-filters

Is SearchPress compatible with Elasticsearch 5.5.3? Any other debugging I can do?