Use UTC timestamp to sign request.

Requests are currently signed with local time but Amazon expects UTC time, which can cause errors like:

```xml
<?xml version="1.0"?>
<ErrorResponse xmlns="http://cloudfront.amazonaws.com/doc/2012-05-05/">
  <Error>
    <Type>Sender</Type>
    <Code>RequestExpired</Code>
    <Message>Request has expired. Time stamp date is Thu, 01 Aug 2013 18:24:43 EST.</Message>
  </Error>
  <RequestId>d2009762-fa83-11e2-b688-298917308a3e</RequestId>
</ErrorResponse>
```