Update Timecop dates to bypass weird test bug

When running the tests locally, I had two tests fail due to an issue
with Timecop. A similar issue has been opened on Timecop's repo, but it
has not been addressed:
travisjeffery/timecop#100

It seems that using a string in Timecop.freeze fixes this issue. This
pull request is opened for conversation and direction on the best way to
resolve the issue.

test/unit/carriers/fedex_test.rb

@@ -39,7 +39,7 @@ def test_home_delivery_business_days
   def test_turn_around_time_default
     mock_response = xml_fixture('fedex/ottawa_to_beverly_hills_rate_response').gsub('<v6:DeliveryTimestamp>2011-07-29</v6:DeliveryTimestamp>', '')
 
-    today = DateTime.civil(2013, 3, 11, 0, 0, 0, "-4") #Monday
+    today = "2013-03-11" #Monday
 
     Timecop.freeze(today) do
       delivery_date = Date.today + 7.days # FIVE_DAYS in fixture response, plus weekend
@@ -278,7 +278,7 @@ def test_delivery_date_from_ground_home_transit_time
 
     @carrier.expects(:commit).returns(mock_response)
 
-    today = DateTime.civil(2015, 06, 04, 0, 0, 0, "-4") #Thursday
+    today = "2015-06-04" #Thursday
 
     Timecop.freeze(today) do
       rate_estimates = @carrier.find_rates( location_fixtures[:ottawa],