Update MappingKM.restart_kernel to accept now kwarg

[vidartf]

This should bring it in line with the signature of the base class (added in jupyter_client 4.0.0).

Fixes #402.

[kevin-bates]

It occurred to me that this could theoretically break subclasses of MKM that override restart_kernel(), but that would require it be called with an explicit now parameter. Since this method is only called from the handler and the handler lets now be conveyed via its default, this is not a practical problem.

While we're at it, would you mind removing the unused **kwargs parameter from GatewayManager's override of this method? I'll be responsible for any issues that might cause, although the same argument holds there, only the handler should be calling that method (when --gateway-url is in play). If you'd rather not, no worries.

[kevin-bates]

Thanks @vidartf - sorry about the delay!