Lets Encrypt with Certbot on Raspbian Jessie

sudo nano /etc/apt/sources.list

deb http://ftp.debian.org/debian jessie-backports main

gpg --keyserver pgpkeys.mit.edu --recv-key 8B48AD6246925553
gpg -a --export 8B48AD6246925553 | sudo apt-key add -

gpg --keyserver pgpkeys.mit.edu --recv-key 7638D0442B90D010
gpg -a --export 7638D0442B90D010 | sudo apt-key add -

sudo a2enmod ssl

sudo apt-get update

sudo apt-get install python-certbot-apache -t jessie-backports -y

sudo certbot --apache certonly

When not working, this may work instead:
sudo certbot certonly --authenticator standalone --installer apache --pre-hook "systemctl stop apache2" --post-hook "systemctl start apache2"
#https://community.letsencrypt.org/t/solution-client-with-the-currently-selected-authenticator-does-not-support-any-combination-of-challenges-that-will-satisfy-the-ca/49983/15


#References
#https://letsencrypt.org/getting-started/
#https://certbot.eff.org/#debianjessie-apache
#https://backports.debian.org/Instructions/
#http://raspberrypi.stackexchange.com/questions/12258/where-is-the-archive-key-for-backports-debian-org