The 'time' library must be explicitly required for 'to_datetime'

to_datetime is currently being used in subscription_helpers.rb and the
specs. Because it isn't required, the specs fail for me. I'm guessing a
gem included it as a side-effect previously, but that's no longer the
case.

Author: pwim

lib/stripe_mock.rb

@@ -1,6 +1,7 @@
 require 'ostruct'
 require 'multi_json'
 require 'dante'
+require 'time'
 
 require 'stripe'