boto3: boto3 1.9.199 polly synthesize_speech doesn't accept Engine parameter

Issue

I was trying Polly neural voices. On Boto3 Polly document, synthesize_speech method already includes Engine parameter.

However, when I used it, it complains about Unknown parameter. Unknown parameter in input: “Engine”, must be one of: LexiconNames, OutputFormat, SampleRate, SpeechMarkTypes, Text, TextType, VoiceId, LanguageCode

Steps

I tried using the following code

print('boto3 ver: ', boto3.__version__)
pollyClient = boto3.client('polly')
try:
    response = pollyClient.synthesize_speech(
                                        Engine='neural',
                                        OutputFormat='mp3',
                                        Text='test',
                                        TextType='ssml',
                                        VoiceId='Matthew'
                                    )
except Exception as e:
    print('synthesize_speech exception: ', e)

boto3 ver:  1.9.199
synthesize_speech exception:  Parameter validation failed:
Unknown parameter in input: "Engine", must be one of: LexiconNames, OutputFormat,
SampleRate, SpeechMarkTypes, Text, TextType, VoiceId, LanguageCode

Does version 1.9.199 support neural voices?