Use unfreeze_time instead of travel_back

This is consistent with `with_frozen_time`.

spec/models/poll/stats_spec.rb

@@ -267,7 +267,7 @@ describe Poll::Stats do
 
         expect(stats.version).to eq "v#{time.to_i}"
 
-        travel_back
+        unfreeze_time
 
         travel_to 2.seconds.from_now do
           expect(stats.version).to eq "v#{time.to_i}"

spec/system/polls/voter_spec.rb

@@ -176,7 +176,7 @@ describe "Voter" do
           expect(page).not_to have_link(answer_yes.title)
         end
 
-        travel_back
+        unfreeze_time
 
         click_link "Sign out"