[tests] Update Travis tests configuration
[tests] Update Travis tests configuration

- Move all tests and sources to Ubuntu Xenial
  - all tested Python versions are supported
  - Xenial is going to be the default environment in a week
- Remove sudo handling as it is not needed anymore
- Move Python 3.7 to the matrix
- No need to specify env in allow failures

Change-Id: Ib9448ee58f4949512b098c78a3299b823c8c8234

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core