WARNING: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY E9C74FEEA2098A6E

Waiting for a dotdeb-keyring package, you just have to get the key and add it to your trusted keys’ keyring :

gpg --keyserver keys.gnupg.net --recv-key E9C74FEEA2098A6E
gpg -a --export E9C74FEEA2098A6E | apt-key add -

I hope you’ll enjoy it.