Tempfile fix

This was originally @tylerhunt's tempfile-fix branch.  I rebased this off the swipely/docker-api master branch.

I then set it up to use `Dir::Tmpname`, so we're not at the whim of the Ruby GC.  The reasoning behind `Tempfile` in the first place was to have a random name.  Since we're using `FileUtils.rm`, we're voiding that part of `Tempfile`.

This should work all the way through

https://github.com/swipely/docker-api/pull/144
Nope, I just would have left it in to prevent regressions.