Fix Onet to work with Go 1.15

This is the first release of Onet that works with Go 1.15.x. If you are using Go 1.15, you must upgrade Onet to this version.

Go 1.15 introduced stricter rules for certificates, and as a result the handshake technique Onet used broke. This release introduces a backwards-compatible change to the handshake technique, so that we can send more public keys, and do it in a way that will not put us at risk of future certificate checking problems. For more info, see #655.

In addition, Onet now depends on kyber v3.0.13, which introduces stricter public key checking during signature verification.