Add GitLab CI config

.gitlab-ci.yml

+image: "ruby:2.3"
+
+# Cache gems in between builds
+cache:
+  paths:
+    - vendor/ruby
+
+before_script:
+  - ruby -v
+  - bundle install -j $(nproc) --path vendor
+
+rubocop:
+  script:
+  - rubocop
+
+assets:
+  script:
+  - rake assets:precompile