gitlab-ci: fix UTF-8 support

.gitlab-ci.yml

@@ -2,6 +2,9 @@ image: ruby:2.5
 stages:
   - build
 
+variables:
+  LANG: "C.UTF-8"
+
 before_script:
   - gem install bundler
 
@@ -9,6 +12,7 @@ build_site:
   stage: build
   only:
     - master
+    - merge_requests
   script:
     - bundle install
     - bundle exec jekyll build