Undocumented CloudFront ViewerCertificate Property: MinimumProtocolVersion

I had the same problem as described [here](https://forums.aws.amazon.com/thread.jspa?messageID=605856): when building a cloudfront distribution with a custom ssl-certificate, you get the error:

> The parameter ViewerCertificate the specified distribution is configured for SNI and the Minimum SSL Protocol Version is specified as SSLv3. You cannot specify both.

As described in that post, editing the template, so the ViewerCertificate block looks like

```
ViewerCertificate: {
IamCertificateId: 'abc1234',
SslSupportMethod: 'sni-only',
MinimumProtocolVersion: 'TLSv1'
}
```

fixes this problem.

I know this is not the fault of troposphere, but is it possible to add `'MinimumProtocolVersion': (basestring, False)` to the ViewerCertificate props, while we wait on AWS to update their documentation?