Don't overwrite api_version argument at DynamoDB.new

Currently, `:api_version` is overwritten with `'2011-12-05'` forcefully at `DynamoDB.new`.

```ruby
pry(main)> AWS::DynamoDB.new(dynamo_db: {api_version: '2012-08-10'}).client
=> #<AWS::DynamoDB::Client::V20111205>
```

I guess it's expected to return a client which is specified by `:api_version` argument.