AIOHttpConnection init raises AttributeError on use_ssl with Elasticsearch>2.0.0

```
aioelasticsearch==0.5.2
elasticsearch==2.4.1
```

`AIOHttpConnection` takes `use_ssl` argument and passes one to the parent's `__init__`.

Then `self.use_ssl` gets used instead of `use_ssl` here
https://github.com/aio-libs/aioelasticsearch/blob/master/aioelasticsearch/connection.py#L52

It works for the latest `elasticsearch-py` versions, however, it doesn't work with `>2.0.0`, b/c there's no `self.use_ssl = use_ssl` there.

The fix is straightforward - use `use_ssl` directly instead of `self.use_ssl`.