Use snap instead of certbot-auto in travis CI (#357)
Use snap instead of certbot-auto in travis CI (#357)

Use snap to install certbot, instead of installing it from source.
This is necessary because certbot-auto has been deprecated
and removed from the certbot source. As a knock-on effect,
because we are no longer activating the certbot venv, directly
install the two python packages (acme and josepy) that chisel2
relies on.

Fixes #354
letsencrypt/pebble,letsencrypt/pebble,letsencrypt/pebble