Upgrade Middleman template for GitLab CI

vendor/gitlab-ci-yml/Pages/Middleman.gitlab-ci.yml

 # Full project: https://gitlab.com/pages/middleman
-image: ruby:2.3
+image: ruby:2.4
+variables:
+  LANG: "C.UTF-8"
 
 cache:
   paths:
   - vendor
 
-test:
-  script:
+before_script:
   - apt-get update -yqqq
   - apt-get install -y nodejs
   - bundle install --path vendor
+
+test:
+  script:
   - bundle exec middleman build
   except:
     - master
 
 pages:
   script:
-  - apt-get update -yqqq
-  - apt-get install -y nodejs
-  - bundle install --path vendor
   - bundle exec middleman build
   artifacts:
     paths: