Remove core Tempfile dependency (1.9.2-preview1 compat)

Remove core Tempfile dependency (1.9.2-preview1 compat)

With the 1.9.2preview1 release (and presumably 1.9.1 p243), the
Ruby core team has decided that bending over backwards to
support crippled operating/file systems was necessary and that
files must be closed before unlinking.

Regardless, this is more efficient than using Tempfile because:

  1) no delegation is necessary, this is a real File object

  2) no mkdir is necessary for locking, we can trust O_EXCL
     to work properly without unnecessary FS activity

  3) no finalizer is needed to unlink the file, we unlink
     it as soon as possible after creation.