TravisCI: migrate from Ubuntu 14.04 (Trusty) to 16.04 (Xenial)

[php-coder]

See for details: https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment

[php-coder]

Installing oraclejdk8
$ export JAVA_HOME=~/oraclejdk8
$ export PATH="$JAVA_HOME/bin:$PATH"
$ ~/bin/install-jdk.sh --target "/home/travis/oraclejdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "BCL"
install-jdk.sh 2019-04-17
Expected feature release number in range of 9 to 13, but got: 8
The command "~/bin/install-jdk.sh --target "/home/travis/oraclejdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "BCL"" failed and exited with 3 during .

It seems like Oracle JDK8 isn't supported anymore.

[php-coder]

A new error from shellcheck has appeared:

src/main/scripts/ci/check-build-and-verify.sh:159:17: warning: Prefer mapfile or read -a to split command output (or quote to avoid splitting). [SC2207]

This because shellcheck has been updated: shellcheck_0.3.3-1~ubuntu14.04.1_amd64.deb -> shellcheck_0.3.7-5_amd64.deb