Generate Rubocop todo file

Generate Rubocop todo file from latest version of Rubocop. Latest
version of Rubocop doesn't support Ruby 2.2 (EOL June 2018), so this
also updates the Ruby dependency going forward to be Ruby 2.3+.

Rubocop todo files help facilitate addressing a large number of style
violations by allowing documented violations. The strategy of a todo
file is to incrementally address violations while preventing violations
of new code.

.rubocop.yml

@@ -1,16 +1,22 @@
+inherit_from: .rubocop_todo.yml
+
+Layout/SpaceBeforeBlockBraces:
+  Enabled: false
+
+# NOTE: Temporarily duplicated from `rubocop_todo.yml` because many files exceed
+#       120 characters
+# Metrics/LineLength:
+#   Max: 120
+
 Naming/PredicateName:
   NameWhitelist:
     - is_enabled?
 
-Metrics/LineLength:
-  Max: 120
-
-Style/RedundantSelf:
-  Enabled: false
 Style/PreferredHashMethods:
   Enabled: false
-Style/StringLiterals:
+
+Style/RedundantSelf:
   Enabled: false
 
-Layout/SpaceBeforeBlockBraces:
+Style/StringLiterals:
   Enabled: false