Use UTC time for 'expires' field in policy, and shorten the duration (for security)

Love the package, this is a small fix.  I was experiencing Policy Expired errors from AWS.  I'm in Central time (UTC-6hrs).  Amazon is expecting an 'expires' field in UTC.  The old code would use local time, and add 5 hrs, which would work for Europe and East Cost US, but would fail elsewhere.