Avoid seasonal clock changes issues in specs

Using `travel` we go to `Time.now + interval`. If the application's time
zone changes due to seasonal clock changes during that interval, we
might travel to a time which is not the time we intended to travel to.

Example:

On a system using the UTC time zone, it's 9AM on October 25 (Friday).
Since by default CONSUL uses the Madrid time zone, it means the
application's time is 11AM.

We use `travel` to advance three days. That means we go to 9AM UTC on
October 28 (Monday). The application's time will be 10AM due to the
seasonal clock change, so we haven't fully traveled three days in
application's time.

To reproduce locally, run:

```
TZ=UTC rspec spec/system/proposal_notifications_spec.rb:410
```

Using `travel_to` with `3.days.from_now`, which uses the application's
time zone and so it will travel to October 28 at 11AM, solves the
problem.

spec/system/proposal_notifications_spec.rb

@@ -405,7 +405,7 @@ describe "Proposal Notifications" do
 
       expect(page).to have_content "Your message has been sent correctly."
 
-      travel(3.days + 1.second) do
+      travel_to(3.days.from_now + 1.second) do
         visit new_proposal_notification_path(proposal_id: proposal.id)
         fill_in "Title", with: "Thank you again for supporting my proposal"
         fill_in "Message", with: "Please share it again with others so we can make it happen!"