Fix 'Requires installation of untrusted packages' error in Ubuntu

1. Terminal run command as following:

sudo apt-get update

2. Find warning :

Get:93 http://archive.ubuntu.com precise-backports/universe Translation-en [26.8 kB]
Fetched 4,954 kB in 12s (387 kB/s)                                             
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EAA903E3A2F4C039

3. Add the pablic key:

sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com  EAA903E3A2F4C039

4. Update again:

sudo apt-get update

5. Check update and install.