Simplify test to edit a poll

Instead of having to add `beginning_of_minute` to deal with an issue
with Capybara filling datetime fields as mentioned in commit 5a0fde4048,
we can travel to the beginning of the minute so we don't have to take
the seconds into account.

spec/system/admin/poll/polls_spec.rb

@@ -75,8 +75,8 @@ describe "Admin polls", :admin do
   end
 
   scenario "Edit" do
-    travel_to(Time.zone.local(2015, 7, 15, 13, 32, 13))
-    poll = create(:poll, :with_image, ends_at: 1.month.from_now.beginning_of_minute)
+    travel_to(Time.zone.local(2015, 7, 15, 13, 32, 00))
+    poll = create(:poll, :with_image, ends_at: 1.month.from_now)
 
     visit admin_poll_path(poll)
     click_link "Edit poll"