Add xenial and version 8.0 support to MySQL sources

[BanzaiMan]

No description provided.

[BanzaiMan]

Tested this config with:

dist: xenial
addons:
  apt:
    sources:
      - sourceline: "deb http://repo.mysql.com/apt/ubuntu/ xenial mysql-8.0"
        key_url: https://raw.githubusercontent.com/travis-ci/apt-source-safelist/master/keys/mysql-5.7-trusty.asc
    packages:
      - mysql-community-client
      - mysql-community-server
      - mysql-community-client-core
      - mysql-community-server-core
      - mysql-client
      - mysql-server
      - mysql-common
      - libmysqlclient-dev
      - libmysqlclient21
      - libstdc++6 
      - libssl1.0.0
      - libmecab2
      - init-system-helpers

but certain dependencies are not resolved.

https://travis-ci.org/BanzaiMan/travis_production_test/builds/454797156#L421-L447

$ sudo -E apt-get -yq --no-install-suggests --no-install-recommends $(travis_apt_get_options) install mysql-community-client mysql-community-server mysql-community-client-core mysql-community-server-core mysql-client mysql-server mysql-common libmysqlclient-dev libmysqlclient21 libstdc++6 libssl1.0.0 libmecab2 init-system-helpers
Reading package lists...
Building dependency tree...
Reading state information...
init-system-helpers is already the newest version (1.14ubuntu1).
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
 libmysqlclient21 : Depends: libssl1.0.0 (>= 1.0.2~beta3) but 1.0.1f-1ubuntu2.26 is to be installed
                    Depends: libstdc++6 (>= 5.2) but 4.8.4-2ubuntu1~14.04.4 is to be installed
 mysql-community-client : Depends: libssl1.0.0 (>= 1.0.2~beta3) but 1.0.1f-1ubuntu2.26 is to be installed
                          Depends: libstdc++6 (>= 5.2) but 4.8.4-2ubuntu1~14.04.4 is to be installed
 mysql-community-client-core : Depends: libssl1.0.0 (>= 1.0.2~beta3) but 1.0.1f-1ubuntu2.26 is to be installed
                               Depends: libstdc++6 (>= 5.2) but 4.8.4-2ubuntu1~14.04.4 is to be installed
                               Depends: libtinfo5 (>= 6) but 5.9+20140118-1ubuntu1 is to be installed
 mysql-community-server : Depends: libmecab2 (>= 0.996-1.2ubuntu1) but 0.996-1.1 is to be installed
                          Depends: libssl1.0.0 (>= 1.0.2~beta3) but 1.0.1f-1ubuntu2.26 is to be installed
                          Depends: libstdc++6 (>= 5.2) but 4.8.4-2ubuntu1~14.04.4 is to be installed
                          Depends: init-system-helpers (>= 1.18~) but 1.14ubuntu1 is to be installed
 mysql-community-server-core : Depends: libmecab2 (>= 0.996-1.2ubuntu1) but 0.996-1.1 is to be installed
                               Depends: libssl1.0.0 (>= 1.0.2~beta3) but 1.0.1f-1ubuntu2.26 is to be installed
                               Depends: libstdc++6 (>= 5.2) but 4.8.4-2ubuntu1~14.04.4 is to be installed
E: Unable to correct problems, you have held broken packages.

[BanzaiMan]

The Trusty job, on the other hand, appears to work.