Enable Jenkins to show graphs properly.

Install `ttf-dejavu` package,
in order to enable Jenkins to show graphs properly.

See also:

- [Jenkins got java.awt.headless problem - Jenkins - Jenkins Wiki](https://wiki.jenkins-ci.org/display/JENKINS/Jenkins+got+java.awt.headless+problem)

This commit fixes #256.
Merge branch 'id/256' into dev
Do not install Docker inside Docker container, while CI testing.

Prerequisite for installing Docker is: update Linux kernel,
but updating Linux kernel is not automated.

Fow workaround to this problem,
do not install Docker inside Docker container, while CI testing.

This commit fixes #257.
Merge branch 'id/257' into dev
Revert "Do not install Docker inside Docker container, while CI testing."

This reverts commit 1cc6045dbaac3025bc46256d8e3be19b1b176d2e,
because Chef seems not to interpret `ENV['DOCKER'].nil?` in its recipes.

This commit refs #257.
Do not install Docker inside Docker container,  while CI testing.

Prerequisite for installing Docker is: update Linux kernel,
but updating Linux kernel is not automated.

Fow workaround to this problem,
do not install Docker inside Docker container, while CI testing.

This commit fixes #257.
Merge branch 'id/257' into dev