Cache bundle files in order to speed up page generation

From 3min-6min to 30sec.

.gitlab-ci.yml

@@ -6,9 +6,15 @@ image: ruby:2.4
 variables:
   JEKYLL_ENV: production
 
+# Cache gems in between builds
+cache:
+  paths:
+    - vendor/ruby
+
 before_script:
+- ruby -v                                   # Print out ruby version for debugging
 - gem install bundler -v '2.0.2'
-- bundle install
+- bundle install -j $(nproc) --path vendor
 
 test:
   stage: test