travis.yml: upgrade to Xenial

Older Ubuntus cause Mercurial connections to fail with this error:

Error on cloning hg repo: unable to get repository: warning:
connecting to bitbucket.org using legacy security technology (TLS
1.0); see https://mercurial-scm.org/wiki/SecureConnections for more
info
(using CA certificates from /etc/ssl/certs/ca-certificates.crt; if you
see this message, your Mercurial install is not properly configured;
see https://mercurial-scm.org/wiki/SecureConnections for how to
configure Mercurial to avoid this message)

Per
https://travis-ci.community/t/travis-ci-linux-build-environments-cant-clone-from-bitbucket-anymore-mercurial/1182/3,
the recommendation is to upgrade to Xenial, which has either a newer
Mercurial, a newer Python, a newer OpenSSL or some combination of
those.

.travis.yml

@@ -1,5 +1,6 @@
 language: go
 sudo: false
+dist: xenial
 notifications:
   email: false
 jobs: