Force UTC last_modified time on rackspace cloudfiles get_container

Re: issue #1809

This is probably not the ideal fix but it will at least show the problem:

From the Rackspace cloudfiles API, get_container returns last_modified values like this:

`2013-05-09T22:20:59.287990`

When doing a `Time.parse` on this value, it is returning the wrong value because it is adding an offset equal to your local time zone. However, the rackspace time is UTC. 

I 'fixed' this by force adding a 'Z' at the end of the last_modified time, which correctly parses the time as UTC.