bug: undefined method [] for nil class in line 69

```
NoMethodError: undefined method `[]' for nil:NilClass
        from /var/lib/gems/1.9.1/gems/speech2text-0.3.6/lib/speech/audio_to_text.rb:69:in `convert_chunk'
        from /var/lib/gems/1.9.1/gems/speech2text-0.3.6/lib/speech/audio_to_text.rb:34:in `block in to_json'
        from /var/lib/gems/1.9.1/gems/speech2text-0.3.6/lib/speech/audio_to_text.rb:32:in `each'
        from /var/lib/gems/1.9.1/gems/speech2text-0.3.6/lib/speech/audio_to_text.rb:32:in `to_json'
        from /var/lib/gems/1.9.1/gems/speech2text-0.3.6/lib/speech/audio_to_text.rb:20:in `to_text'
        from (irb):14
        from /usr/bin/irb:12:in `<main>'
```