Use appropriate maybe-future to handle asyncio futures

[kevin-bates]

The method to handle conditional returns of futures in tornado was both deprecated and incompatible with asyncio. This incompatibility surfaces with Notebook 6.1 since it uses asyncio. Now using maybe_future from notebook.utils instead.

Updated the python matrix in Travis.

Fixes #337

[lresende]

LGTM, we should really look into transitioning users to EG which has been merged with JKG and supports both local and remote kernels.

[kevin-bates]

Thanks for the review @lresende.

we should really look into transitioning users to EG which has been merged with JKG and supports both local and remote kernels

True, but EG doesn't support the http personality: --KernelGatewayApp.api='kernel_gateway.notebook_http' so JKG is still necessary for that.