Allow current way to write respond_to blocks

Not sure exactly since when, but we've started to get rubocop failures
when using the pattern:

respond_to do |format|
  format.js
end

We've been using this pattern for ages, so maybe a recent version of
Rubocop introduced a change that made it report it.

In any case, this is easier to read than respond_to(&:js), so we're
allowing it.

.rubocop.yml

@@ -594,6 +594,8 @@ Style/StringLiteralsInInterpolation:
 
 Style/SymbolProc:
   Enabled: true
+  AllowedMethods:
+    - respond_to
 
 Style/TrailingCommaInArrayLiteral:
   Enabled: true