Bump 37/ZMTP version to 4.0

[ptitjes]

The protocol specified in 37/ZMTP presents breaking changes in binary compatibility. (Subscribe/Unsubscribe messages versus Subscribe/Cancel commands.)

The protocol specifies version downgrading at the level of the major version (cf. the first three paragraphs of the version negociation.

Therefore, I believe the version of the protocol should be bumped to 4.0.