Tempfile not deleted when using "format"

I'm using 3.7.0.
Minimagick leaves an additional tempfile in /tmp when using the "format" command.

```ruby
# test.rb

require "mini_magick"

image = MiniMagick::Image.open("test.tif")
image.format "jpg"
```

such that my disk fills up.

Before i run `ruby test.rb`: `ls /tmp/mini_magick*` gives 0 results
Afterwards, i have a /tmp/mini_magick20131127-18504-ula55j.jpg that doesn't get deleted when the script terminates, etc.