To import the key, open a terminal and enter these commands

sudo gpg --keyserver hkp://subkeys.pgp.net --recv-keys 16126D3A3E5C1192 

You would see the following output if the above is successful

gpg: Total number processed: 1 gpg: imported: 1 

Then run this command:

sudo gpg --export --armor 16126D3A3E5C1192 | sudo apt-key add - 

Note the - sign after add.

Then sudo apt-get update, you will have no such messages after this.