.travis.yml: Configure xenial as the default environment...
.travis.yml: Configure xenial as the default environment...

...as this is required for versions of python >= 3.7
Also remove 'sudo: true' as this no longer needed, according to:
https://github.com/travis-ci/travis-ci/issues/9815

(This is a follow-up to commit e8d135b.)

asqui/psycopg-nestedtransactions