Update the image to focal

[aajisaka]

Use Focal: https://docs.travis-ci.com/user/reference/focal

[aajisaka]

https://travis-ci.org/github/aajisaka/hadoop-document/builds/769293553

Installing openjdk8
$ export JAVA_HOME=~/openjdk8
$ export PATH="$JAVA_HOME/bin:$PATH"
$ ~/bin/install-jdk.sh --target "/home/travis/openjdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "GPL" --cacerts
Ignoring license option: GPL -- using GPLv2+CE by default
install-jdk.sh 2020-06-02
Expected feature release number in range of 9 to 17, but got: 8
The command "~/bin/install-jdk.sh --target "/home/travis/openjdk8" --workspace "/home/travis/.cache/install-jdk" --feature "8" --license "GPL" --cacerts" failed and exited with 3 during .

[aajisaka]

It is broken: travis-ci/docs-travis-ci-com#2902

Trying workaround...

[aajisaka]

After #5, GitHub Actions is used instead of Travis CI.