Bump ed25519 dependency to v2

Release notes: RustCrypto/signatures#622

Author: tarcieri

Cargo.lock

@@ -25,7 +25,7 @@ features = ["nightly", "batch", "pkcs8"]
 
 [dependencies]
 curve25519-dalek = { version = "=4.0.0-pre.5", default-features = false, features = ["digest"] }
-ed25519 = { version = "=2.0.0-rc.0", default-features = false }
+ed25519 = { version = "2", default-features = false }
 merlin = { version = "3", default-features = false, optional = true }
 rand_core = { version = "0.6.4", default-features = false, optional = true }
 serde = { version = "1.0", default-features = false, optional = true }