Update travel_to usage

In Rails 5.1, calling `travel_to` inside another `travel_to` block will
result in a RuntimeError:

> Calling `travel_to` with a block, when we have previously already made
> a call to `travel_to`, can lead to confusing time stubbing.

spec/features/polls/voter_spec.rb

@@ -187,20 +187,16 @@
  expect(page).not_to have_link(answer_yes.title)
  end
 
+ travel_back
+
  click_link "Sign out"
 
- # Time needs to pass between the moment we vote and the moment
- # we log in; otherwise the link to vote won't be available.
- # It's safe to advance one second because this test isn't
- # affected by possible date changes.
- travel 1.second do
- login_as user
- visit poll_path(poll)
-
- within("#poll_question_#{question.id}_answers") do
- expect(page).to have_link(answer_yes.title)
- expect(page).to have_link(answer_no.title)
- end
+ login_as user
+ visit poll_path(poll)
+
+ within("#poll_question_#{question.id}_answers") do
+ expect(page).to have_link(answer_yes.title)
+ expect(page).to have_link(answer_no.title)
  end
  end
  end

spec/models/poll/stats_spec.rb

@@ -271,6 +271,8 @@
 
  expect(stats.version).to eq "v#{time.to_i}"
 
+ travel_back
+
  travel_to 2.seconds.from_now do
  expect(stats.version).to eq "v#{time.to_i}"
  end