Update CA certificates to latest Mozilla bundle

Copied from an up-to-date Fedora 27 install (ca-certificates-2018.2.22-1.0.fc27).

(cherry picked from commit 294af5617a9062f3d9d0637a65db4a0af4675892)

thirdparty/README.md

@@ -10,9 +10,14 @@
 
 ## certs
 
-- Upstream: ?
-
-TODO.
+- Upstream: Mozilla, via https://apps.fedoraproject.org/packages/ca-certificates
+- Version: 2018.2.22
+- License: MPL 2.0
+
+File extracted from a recent Fedora install:
+/etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem
+(It can't be extracted directly from the package,
+as it's generated on the user's system.)
 
 
 ## fonts