Use Ubuntu Xenial (16.04) at Travis CI
Use Ubuntu Xenial (16.04) at Travis CI

https://docs.travis-ci.com/user/reference/xenial/

* MySQL 5.7 and PostgreSQL 10 is available by default
https://docs.travis-ci.com/user/reference/xenial/#databases-and-services

* No matrixes necessary for "GEM=activerecord:mysql2", "GEM=activerecord:postgresql", "GEM=railties"
and "GEM=actionmailer,activemodel,activesupport,actionview,activejob,activestorage,actionmailbox,actiontext"

* Run mysql_upgrade for MariaDB

* Increase ramfs size before install for "GEM=activerecord:postgresql"

* Use FFMpeg 3.x using ppa:jonathonf/ffmpeg-3

    Ubuntu 16.04 installs FFMpeg version 2.x by default, which
    does not pass two of Active Stroage tests reported at https://github.com/rails/rails/issues/34921

* Use `rabbitmq-server` package and service

* Update bundler and rubygems to address "Could not find 'bundler' (1.17.2)" for ruby-head

    ```
    $ bundle install --jobs 3 --retry 3
    Traceback (most recent call last):
            2: from /home/travis/.rvm/rubies/ruby-head/bin/bundle:30:in `<main>'
            1: from /home/travis/.rvm/rubies/ruby-head/lib/ruby/2.7.0/rubygems.rb:302:in `activate_bin_path'
    /home/travis/.rvm/rubies/ruby-head/lib/ruby/2.7.0/rubygems.rb:283:in `find_spec_for_exe': Could not find 'bundler' (1.17.2) required by your /home/travis/build/rails/rails/Gemfile.lock. (Gem::GemNotFoundException)
    To update to the latest version installed on your system, run `bundle update --bundler`.\nTo install the missing version, run `gem install bundler:1.17.2`\n
    ```

* Use oraclejdk11 (18.9 LTS) to address the following error

    ```
    $ ~/bin/install-jdk.sh --target "/home/travis/oraclejdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "BCL"
    install-jdk.sh 2018-10-17
    Expected feature release number in range of 9 to 13, but got: 8
    ```

    Refer:
    https://docs.travis-ci.com/user/reference/xenial/#jvm-clojure-groovy-java-scala-support

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