Switch to tempfile instead of tempdir

[emoon]

This is because tempdir has been deprecated with this info from cargo-deny

31 │ tempdir 0.3.7 registry+https://github.com/rust-lang/crates.io-index
   │ ------------------------------------------------------------------- unmaintained advisory detected
   │
   = ID: RUSTSEC-2018-0017
   = Advisory: https://rustsec.org/advisories/RUSTSEC-2018-0017
   = The [`tempdir`](https://crates.io/crates/tempdir) crate has been deprecated
     and the functionality is merged into [`tempfile`](https://crates.io/crates/tempfile).
   = Announcement: https://github.com/rust-lang-deprecated/tempdir/pull/46
   = Solution: No safe upgrade is available!
   = tempdir v0.3.7
     └── dynamic_reload v0.8.0
         └── rv_core v0.1.0