ruby: include Rubocop

We use a fair bit of Ruby logic here and there so until this website
disappears (not soon), we might as well ensure we're hacking as one.

.rubocop.yml

@@ -0,0 +1,18 @@
+inherit_from: .rubocop_todo.yml
+
+# The behavior of RuboCop can be controlled via the .rubocop.yml
+# configuration file. It makes it possible to enable/disable
+# certain cops (checks) and to alter their behavior if they accept
+# any parameters. The file can be placed either in your home
+# directory or in some project directory.
+#
+# RuboCop will start looking for the configuration file in the directory
+# where the inspected file is and continue its way up to the root directory.
+#
+# See https://docs.rubocop.org/rubocop/configuration
+require:
+  - rubocop-rspec
+  - rubocop-rake
+
+AllCops:
+  NewCops: enable