updated oracle-cert.cer for VirtualBox 4.3.8

Building packer boxes with VirtualBox 4.3.8 the vbox additions are not installed silently. This is because the oracle-cert.cer expired on February 2014, but still can be used installing the vbox additions until 4.3.6. Here is the new certificate for vbox additions 4.3.8 and above.
But this could be tricky to make all users happy with different versions of VirtualBox installed on their hosts.