travis: Run tests on 16.04 for Python 3.7, 3.8-dev

[kba]

With this change, tests on Travis will run on 16.04 containers instead of 14.04 (for which 3.7 and 3.8-dev seem unavailable).

c.f. https://travis-ci.org/kba/bagit-python/builds/464906319

BTW The coveralls setup appears broken, the configuration file has been removed from the repo and there is an error Travis:

Submitting coverage to coveralls.io...
Coverage submitted!
Couldn't find a repository matching this job.
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.7.1/bin/coveralls", line 11, in <module>
    sys.exit(main())
  File "/home/travis/virtualenv/python3.7.1/lib/python3.7/site-packages/coveralls/cli.py", line 80, in main
    log.info(result['url'])
KeyError: 'url'

[acdha]

Coveralls doesn't need the config file. They must have changed something on their side; I'll see if they have any useful diagnostic information.

[acdha]

Oh, that's because it's your build rather than ours — I'm assuming you could register your repo at coveralls.io but otherwise it seems to be working.

[coveralls]

Coverage decreased (-0.05%) to 83.502% when pulling fb6c7b1 on kba:travis-on-xenial into c39b650 on LibraryOfCongress:master.