Cognito event version should be a number

Looking at the [developer guide for lambda triggers](https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-identity-pools-working-with-aws-lambda-triggers.html) it shows version as **a number not a string**.

Libs for other languages, e.g. TypeScript, is using number as well https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/aws-lambda/index.d.ts#L258

Is there any reason why it is a string in Go's version?

I ran into this as I was using to https://github.com/serverless/event-mocks to test my post confirm end-point.