Switching Travis CI builds to Ubuntu Xenial Xerus

[morozov]

Q	A
Type	improvement
BC Break	no

Solved issues:

• PostgreSQL 9.2 service fails to start (not supported).
• PostgreSQL 9.3 service fails to start (not supported).
• MariaDB 10.1 service fails to start (#3151, Don't mark dependency packages as manually installed travis-ci/travis-cookbooks#1057, supported until October 2020).
• PostgreSQL 10.0 fails to start.

Summary of the changes:

1. Request the mysql service explicitly when needed (migration guide).
2. Dockerize MariaDB 10.1 since Travis doesn't give a damn about it and it'll be easier for us to transition somewhere else later.
3. Spell the PostgreSQL 10.0 version as 10 to comply with the naming on Travis.
4. Make sure the travis user exists on a MySQL-like DB since Travis doesn't create it consistently.

[morozov]

The issue solved itself (dotnet/core#2608 (comment)). The build is passing on Trusty Tahr.

[jwage]

Do you still want to merge this?

[morozov]

I want to experiment a bit more later. Trusty is 5 years old and EOL this month.

[morozov]

Closing until https://travis-ci.community/t/mariadb-10-1-fails-to-install-on-xenial/3151 is resolved.