Use CI_COMMIT_REF_SLUG for the doxygen zip file

According to https://docs.gitlab.com/ce/ci/variables/ it's more URL
friendly than CI_COMMIT_REF_NAME.

.gitlab-ci.yml

@@ -35,9 +35,9 @@ doxygen:
     - make BUILDDIR=build doc
     - rm -rf public
     - mkdir -p public/doxygen
-    - mv build/doxygen/html ${CI_COMMIT_REF_NAME}
+    - mv build/doxygen/html ${CI_COMMIT_REF_SLUG}
     - yum install -y zip
-    - zip -r public/doxygen/${CI_COMMIT_REF_NAME}.zip ${CI_COMMIT_REF_NAME}
+    - zip -r public/doxygen/${CI_COMMIT_REF_SLUG}.zip ${CI_COMMIT_REF_SLUG}
   only:
     - master
     - tags