fix(spec): avoid flacky test. 15.days ago is not always on the previous month. eg: this spec failed on 16 of december, stats is still in december

spec/controllers/stats_controller_spec.rb

@@ -1,4 +1,7 @@
 describe StatsController, type: :controller do
+  before { Timecop.travel(Date.parse("2021/12/15")) }
+  after { Timecop.return }
+
   describe "#last_four_months_hash" do
     context "while a regular user is logged in" do
       before do