Follow up: Fix CI failure introduced in 0b8b6148b

The previous commit introduced a CI failure which was caused by a test depending on the execution time. The Twitter channel implementation ignores tweets older than 15 days.
Therefore we need to travel to the time when the VCR snapshot was recorded to fix it.

spec/models/channel_spec.rb

@@ -6,6 +6,9 @@ RSpec.describe Channel do
       context 'with valid token' do
         subject(:twitter_channel) { create(:twitter_channel) }
 
+        # travel back in time when VCR was recorded
+        before { travel_to '2020-02-06 13:37 +0100' }
+
         it 'returns true' do
           expect(twitter_channel.fetch(true)).to be(true)
         end