12.04.4 server can't verify common SSL certificates, usual fixes failing
I also have this problem, try this:
openssl s_client -host google.com -port 443
this command will also print a cert chain, the last one is:
s:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA
i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority
so you also need https://www.geotrust.com/resources/root_certificates/certificates/Equifax_Secure_Certificate_Authority.pem.
This cert path is different with cert path showed in browser (IE, Firefox, Chrome), I don't know why, but this fix my problem.