Like WelshPaul said, this is included in 3.2 by default.
And with 3.1 being at an End of Maintenance
state, we won't add any new features. We could help you upgrade to 3.2, if you want.
Additionally though, there is a 3.1 extension that adds reCAPTCHA 2.0 to your board
. Just be aware though that this is an unvalidated, abandoned extension. That means that there is no support for it and we can't guarantee it's working or security. Proceed at your own risk.