Bugfix: s/LastUpdatedTimestamp/LastUpdatedTime

The cloudformation api is returning xml that does not have
LastUpdatedTimestamp elements, but rather LastUpdatedTime elements. Here
is an example:

<ListStacksResponse xmlns="http://cloudformation.amazonaws.com/doc/2010-05-15/">
  <ListStacksResult>
    <StackSummaries>

...snip

      <member>
      <StackId>STACKY_DOO_DAH</StackId>
      <StackStatus>UPDATE_COMPLETE</StackStatus>
==>   <LastUpdatedTime>2013-08-09T18:58:22Z</LastUpdatedTime>
      <StackName>StackyDooDah</StackName>
      <CreationTime>2013-08-09T15:18:49Z</CreationTime>
      <TemplateDescription>juggernauts</TemplateDescription>
      <StackStatusReason/>
      </member>

This commit fixes this and adjusts a couple of tests accordingly.