Add env to the kernelspec __init__

the env attribute was added to the kernelspec and the kernel.json file, but has an error when you try to use it because it's not in `__init__`
Well (investigating the failing test), for one, having env=None in the default arguments means that `env` gets set to None, not an empty dict.  And then doing `env = Dict(allow_none=False)` in the spec complains that env is None.  On the one hand, we can have `env` in the `**kwargs` and let the traits magic take care of initialization.  On the other hand, we can explicitly check to see if `env` is None and set it to the empty dict.  It's your call on which we should do.