Address rubocop offences

[yuki24]

The Rubocop checks are failing with the latest Rubocop version (e.g. build#74):

Running RuboCop...
RuboCop failed!
Inspecting 63 files
....W..........................................................

Offenses:

features/step_definitions/flatware_steps.rb:34:14: W: [Correctable] Lint/Void: Operator - used in void context.
    Time.now - started_at
             ^

63 files inspected, 1 offense detected, 1 offense autocorrectable

However, when I run bundle exec rubocop -a to auto-fix them, the code ends up with an incorrect fix:

diff --git a/features/step_definitions/flatware_steps.rb b/features/step_definitions/flatware_steps.rb
index 1728508..6899b1f 100644
--- a/features/step_definitions/flatware_steps.rb
+++ b/features/step_definitions/flatware_steps.rb
@@ -28,10 +28,10 @@ module Support
   end

   def duration(&_block)
-    started_at = Time.now
+    Time.now
     yield
   ensure
-    Time.now - started_at
+    Time.now
   end
 end
 World(Support)

I believe that what we want to do with this method is to just measure how long the given block takes to execute, and we should be able to do that with the Benchmark module, which should be more appropriate:

Benchmark.realtime { slow_method }
# => returns the time taken to execute the #slow_method, in float.