Support for longpool

[sezaru]

Looking at the code, it seems that Livebook only support websockets and don't have a longpool fallback.

Why is that?

I'm trying to use Livebook with Google Colab, but their reverse proxy system don't support websockets, so I believe the only option that would work in this case is using longpool.

[josevalim]

We should enable the fallback. Please PR!