I believe you're running into the same problem that's described in this thread:
@joohoi provided a temporary workaround in that thread. Since you're on Debian jessie, you could also install the certbot package (which is the new name for the letsencrypt client, all commands should be compatible) from backports via sudo apt-get install python-certbot-apache -t jessie-backports. More details can be found on the certbot homepage.