Explicitly enable unstable library feature

[pefoley2]

rustc 1.23.0 currently bails with:

error: use of unstable library feature 'refcell_replace_swap' (see issue #43570)
--> src/signature.rs:466:31
|
466 | self.payload_hash.replace(Some([hash[0], hash[1]]));
| ^^^^^^^
|
= help: add #![feature(refcell_replace_swap)] to the crate attributes to enable

error: aborting due to previous error

error: Could not compile pretty-good.

[csssuf]

That feature was stabilized in rust-lang/rust#46517 (released in rustc 1.24.0), and I intend to keep this library targeting stable, so you'll just need to upgrade your Rust.