Binder demo: kernel does not start

[nthiery]

Steps to reproduce:

• Follow the binder link on xcanva's README
• Open a C++ notebook (tested with C++14 and C++17)
• Execute a cell (e.g. with 1+1)
• Execution hangs for a couple seconds, and then the kernel name switches to "No kernel"

The javascript console display messages:

Firefox ne peut établir de connexion avec le serveur à l’adresse wss://hub.gke2.mybinder.org/user/martinrenou-xcanvas-wyutngqq/api/kernels/cb1892ac-1a41-439f-b716-871b000caaba/channels?session_id=9b3e2b4d-b382-4bcd-87a8-0fcf1aa92f92. [jlab_core.716b79be8019e5bdc801.js:2:985464](https://hub.gke2.mybinder.org/user/martinrenou-xcanvas-wyutngqq/static/lab/jlab_core.716b79be8019e5bdc801.js?v=716b79be8019e5bdc801)
Firefox ne peut établir de connexion avec le serveur à l’adresse wss://hub.gke2.mybinder.org/user/martinrenou-xcanvas-wyutngqq/api/kernels/cb1892ac-1a41-439f-b716-871b000caaba/channels?session_id=939fedca-5039-4d52-aa28-b47a23ccaac2. [jlab_core.716b79be8019e5bdc801.js:2:985464](https://hub.gke2.mybinder.org/user/martinrenou-xcanvas-wyutngqq/static/lab/jlab_core.716b79be8019e5bdc801.js?v=716b79be8019e5bdc801)
La connexion avec wss://hub.gke2.mybinder.org/user/martinrenou-xcanvas-wyutngqq/api/kernels/cb1892ac-1a41-439f-b716-871b000caaba/channels?session_id=9b3e2b4d-b382-4bcd-87a8-0fcf1aa92f92 a été interrompue pendant le chargement de la page. [jlab_core.716b79be8019e5bdc801.js:2:985464](https://hub.gke2.mybinder.org/user/martinrenou-xcanvas-wyutngqq/static/lab/jlab_core.716b79be8019e5bdc801.js?v=716b79be8019e5bdc801)
La connexion avec wss://hub.gke2.mybinder.org/user/martinrenou-xcanvas-wyutngqq/api/kernels/cb1892ac-1a41-439f-b716-871b000caaba/channels?session_id=939fedca-5039-4d52-aa28-b47a23ccaac2 a été interrompue pendant le chargement de la page. [jlab_core.716b79be8019e5bdc801.js:2:985464](https://hub.gke2.mybinder.org/user/martinrenou-xcanvas-wyutngqq/static/lab/jlab_core.716b79be8019e5bdc801.js?v=716b79be8019e5bdc801)
XHR

Python's kernel works

Cheers,