Add :none option to :compression_codec

The comment in lib/poseidon/producer.rb says users can set the option `:compression_codec` to `:none`.

```ruby
# @option options [:gzip / :snappy / :none] :compression_codec (:none)
# Type of compression to use.
```
but I got the following error message when I set the option to `:none`.
> Unknown compression codec: 'none' (accepted: [:gzip, :snappy])

This patch enables users to set the option to `:none`.