Merge branch 'ps/ci-depends-on-ruby' into jch

The CI job that builds the documentation failed because it lacked
the 'gem' command. The 'asciidoc' package apparently stopped pulling
in the 'ruby' package as a transitive dependency.  The CI script has
been updated to explicitly install 'ruby'.

* ps/ci-depends-on-ruby:
  ci: fix missing Ruby dependency in "documentation" job

ci/install-dependencies.sh

@@ -164,7 +164,7 @@ sparse)
 		libexpat-dev gettext zlib1g-dev sparse
 	;;
 Documentation)
-	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make
+	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make ruby
 
 	test -n "$ALREADY_HAVE_ASCIIDOCTOR" ||
 	sudo gem install --version 1.5.8 asciidoctor