Validate user has signed Typesafe CLA

[dotta]

Would be handy if the validator could make sure that the user who created the PR has signed the Typesafe CLA. If it hasn't yet, the validator should point the user to the online Typesafe CLA and explain that the PR cannot be accepted until the CLA is signed.

[cvrebert]

That seems to be the job of https://github.com/typesafehub-validator