Enforce standardizing nodoc comments by rubocop

Follow up to 18707ab, b87d8e5.

rubocop 1.19.0 includes rubocop/rubocop#9964.

Author: kamipo

Gemfile.lock

@@ -340,7 +340,7 @@ GEM
       racc (~> 1.4)
     os (1.1.1)
     parallel (1.20.1)
-    parser (3.0.1.1)
+    parser (3.0.2.0)
       ast (~> 2.4.1)
     path_expander (1.1.0)
     pg (1.2.3)
@@ -395,23 +395,23 @@ GEM
     retriable (3.1.2)
     rexml (3.2.5)
     rouge (3.26.0)
-    rubocop (1.18.1)
+    rubocop (1.19.0)
       parallel (~> 1.10)
       parser (>= 3.0.0.0)
       rainbow (>= 2.2.2, < 4.0)
       regexp_parser (>= 1.8, < 3.0)
       rexml
-      rubocop-ast (>= 1.7.0, < 2.0)
+      rubocop-ast (>= 1.9.1, < 2.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 3.0)
-    rubocop-ast (1.7.0)
+    rubocop-ast (1.9.1)
       parser (>= 3.0.1.1)
     rubocop-packaging (0.5.1)
       rubocop (>= 0.89, < 2.0)
-    rubocop-performance (1.11.3)
+    rubocop-performance (1.11.4)
       rubocop (>= 1.7.0, < 2.0)
       rubocop-ast (>= 0.4.0)
-    rubocop-rails (2.11.2)
+    rubocop-rails (2.11.3)
       activesupport (>= 4.2.0)
       rack (>= 1.1)
       rubocop (>= 1.7.0, < 2.0)