gitlab-ci: fix asciidoctor version in opensuse.

openSUSE now ships with 2.0.0 which is incompatible. Use 1.5.8 instead.
Disable ri and rdoc while here.

Change-Id: I40766ae4377f5998dcd58d02ecbdac665f78222a
Reviewed-on: https://code.wireshark.org/review/32547
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>

.gitlab-ci.yml

@@ -93,7 +93,7 @@ rpm-opensuse-42.3:
   script:
     - zypper --non-interactive install update-desktop-files
     - tools/rpm-setup.sh --install-optional ruby
-    - gem install asciidoctor
+    - gem install asciidoctor -v 1.5.8 --no-ri --no-rdoc
     - mkdir build
     - cd build
     - cmake -GNinja ..