Put timecop into safe mode and fix leaky Timecop.freeze

Gemfile.lock

@@ -594,7 +594,7 @@ GEM
       ref
     thor (0.19.1)
     tilt (1.4.1)
-    timecop (0.6.2.2)
+    timecop (0.8.1)
     timers (1.1.0)
     treetop (1.4.15)
       polyglot

config/initializers/timecop.rb

@@ -0,0 +1 @@
+Timecop.safe_mode = true

spec/models/spree/product_spec.rb

@@ -33,9 +33,10 @@ module Spree
       end
 
       it "defaults available_on to now" do
-        Timecop.freeze
-        product = Product.new
-        product.available_on.should == Time.zone.now
+        Timecop.freeze do
+          product = Product.new
+          product.available_on.should == Time.zone.now
+        end
       end
 
       describe "tax category" do