ci: run gem with sudo to install asciidoctor

In a later patch, we will run Documentation job in GitHub Actions.
The job will run without elevated permission.

Run `gem` with `sudo` to elevate permission in order to be able to
install to system location.
This will also keep this installation in-line with other installation in
our Linux system for CI.

Signed-off-by: Johannes Schindelin <[email protected]>
[Danh: reword commit message]
Signed-off-by: Đoàn Trần Công Danh <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

ci/install-dependencies.sh

@@ -70,7 +70,7 @@ Documentation)
 	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make
 
 	test -n "$ALREADY_HAVE_ASCIIDOCTOR" ||
-	gem install --version 1.5.8 asciidoctor
+	sudo gem install --version 1.5.8 asciidoctor
 	;;
 linux-gcc-4.8|GETTEXT_POISON)
 	sudo apt-get -q update