Adding on to unSpawn you can add that certificate to your trust store like so...
Code:
openssl x509 -text -in DigiCertGlobalCA.crt >> /etc/ssl/certs/ca-bundle.trust.crt
Assuming that's how your server handles certificates (it does it that way on my Fedora and similarly Ubuntu machines). Though my server already has the DigiCert CA certs installed by default.
*edit*: on my Ubuntu system the path is
/etc/ssl/certs/ca-certificates.crt. Also you don't need to run that openssl command to import it on Ubuntu; you can simply cat the certificate...
Code:
cat DigiCertGlobalCA.crt >> /etc/ssl/certs/ca-certificates.crt