Fix dev version parsing

Without this, plat claims that no packages are available. For example:

    $ plat versions acme.lab
    acme.lab:

This is because XMLRPCPyPIAPI.get_available_versions effectively does
the following:

    dev = False
    if dev == is_dev_version("1.0.0"):
        # We never get here, because is_dev_version returns None instead
        # of False!
        versions.add("1.0.0")

The fix is straightforward, but I want to see Travis moan first (I'm struggling to run the tests locally).

Assumes #28 will be merged.