USN-5473-2: ca-certificates update

USN-5473-2: ca-certificates update

USN-5473-1 updated ca-certificates. This update provides
the corresponding update for Ubuntu 16.04 ESM.

Original advisory details:

The ca-certificates package contained outdated CA certificates. This update
refreshes the included certificates to those contained in the 2.50 version
of the Mozilla certificate authority bundle.
Source: USN-5473-2: ca-certificates update