TST: Use more accurate time windows so tests do not fail

Currently, the way the 'starts_at' and 'ends_at' attributes are set,
if the specs are run at one second to midnight, the broadcast message
will expire in one second. I have changed it so that we are guaranteed
a period of one day until expiration. I believe this is the desired
behaviour and it's also consistent with the rest of the factory.

This corrects the following three specs that can fail depending upon
where and when they are run:

./spec/helpers/broadcast_messages_helper_spec.rb:42
./spec/models/broadcast_message_spec.rb:26
./spec/models/broadcast_message_spec.rb:47

spec/factories/broadcast_messages.rb

 FactoryGirl.define do
   factory :broadcast_message do
     message "MyText"
-    starts_at Date.yesterday
-    ends_at Date.tomorrow
+    starts_at 1.day.ago
+    ends_at 1.day.from_now
 
     trait :expired do
       starts_at 5.days.ago