Use Ubuntu bionic (18.04) for CI

test/ci-bootstrap.py

@@ -61,7 +61,7 @@ if __name__ == "__main__":
     if args.create_droplet:
         # Create droplet
 
-        # image: 7 = Ubuntu xenial 16.04 x64
+        # image: 18 = Ubuntu xenial 18.04 x64
         # ssh_keys
         # - 411: ci, ed25519
         # - 407: varac