Use Time instead of DateTime

- Active record works better with Time, as opposed to DateTime objects,
  see issue doorkeeper-gem#668 for more details.

lib/doorkeeper/models/concerns/revocable.rb

@@ -1,12 +1,12 @@
 module Doorkeeper
   module Models
     module Revocable
-      def revoke(clock = DateTime)
+      def revoke(clock = Time)
         update_attribute :revoked_at, clock.now
       end
 
       def revoked?
-        !!(revoked_at && revoked_at <= DateTime.now)
+        !!(revoked_at && revoked_at <= Time.now)
       end
     end
   end

spec/lib/models/revocable_spec.rb

@@ -19,12 +19,12 @@
 
   describe :revoked? do
     it 'is revoked if :revoked_at has passed' do
-      allow(subject).to receive(:revoked_at).and_return(DateTime.now - 1000)
+      allow(subject).to receive(:revoked_at).and_return(Time.now - 1000)
       expect(subject).to be_revoked
     end
 
     it 'is not revoked if :revoked_at has not passed' do
-      allow(subject).to receive(:revoked_at).and_return(DateTime.now + 1000)
+      allow(subject).to receive(:revoked_at).and_return(Time.now + 1000)
       expect(subject).not_to be_revoked
     end