Use Tempfile instead of Dir.mktmpdir

It seems that our tempdir disappeared before we could use it. Using a file in
TMPDIR instead means we shouldn't have to worry about that.

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>
Remove unused require from spec

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>