ErrSignatureInvalid from jwt-go lib should be handled by jwt parser

I'm playing around with jwt errors handling and I found out that there is a [HS256-specific error](https://github.com/dgrijalva/jwt-go/blob/dc14462fd58732591c7fa58cc8496d6824316a82/hmac.go#L21) in jwt-go library that is returned as inner error by the go-kit parser. I believe that this error should be handled by the parser and `ErrTokenInvalid` should be returned.

Any thoughts?