Update jubjub requirement from 0.7 to 0.8 (#131)

* Update jubjub requirement from 0.7 to 0.8

Updates the requirements on [jubjub](https://github.com/zkcrypto/jubjub) to permit the latest version.
- [Release notes](https://github.com/zkcrypto/jubjub/releases)
- [Changelog](https://github.com/zkcrypto/jubjub/blob/main/RELEASES.md)
- [Commits](https://github.com/zkcrypto/jubjub/commits/0.8.0)

---
updated-dependencies:
- dependency-name: jubjub
  dependency-type: direct:production
...

Cargo.toml

@@ -21,7 +21,7 @@ features = ["nightly"]
 blake2b_simd = "0.5"
 byteorder = "1.4"
 digest = "0.9"
-jubjub = "0.7"
+jubjub = "0.8"
 rand_core = "0.6"
 serde = { version = "1", optional = true, features = ["derive"] }
 thiserror = "1.0"