Fix UniFi Controller Mongodb 3.4

An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release: The following signatures were invalid: EXPKEYSIG BC711F9BA15703C6 MongoDB 3.4 Release Signing Key W: Failed to fetch http://repo.mongodb.org/apt/ubuntu/dists/xenial/mongodb-org/3.4/Release.gpg The following signatures were invalid: EXPKEYSIG BC711F9BA15703C6 MongoDB 3.4 Release Signing Key W: Some index files failed to download. They have been ignored, or old ones used instead.

wget -qO - https://www.mongodb.org/static/pgp/server-3.4.asc | sudo apt-key add -