Specify default Travis CI build platform explicitly

This doesn't really change anything right now, as Ubuntu Xenial is used
by default anyhow, but removes the last message from the validator at
https://config.travis-ci.com/explore so still worth having.

.travis.yml

@@ -4,6 +4,10 @@
 # root directory of the project.
 language: cpp
 
+# Specify the default platform.
+os: linux
+dist: xenial
+
 jobs:
     include:
         - dist: trusty