jnano: add support for upstream renaming of NN_VERSION_*

The `nanomsg` library introduced the following changes:

    "NN_VERSION_MAJOR" --> "NN_VERSION_CURRENT"
    "NN_VERSION_MINOR" --> "NN_VERSION_REVISION"
    "NN_VERSION_PATCH" --> "NN_VERSION_AGE"

This code adds compatibility to `jnano` and its unit tests, which will now pass with the change added, whereas before they would fail.