spl: Use git url for serum_dex for development (#575)

spl/Cargo.toml

@@ -12,6 +12,6 @@ devnet = []
 [dependencies]
 anchor-lang = { path = "../lang", version = "0.12.0", features = ["derive"] }
 lazy_static = "1.4.0"
-serum_dex = { version = "0.4.0", features = ["no-entrypoint"] }
+serum_dex = { git = "https://github.com/project-serum/serum-dex", version = "0.4.0", features = ["no-entrypoint"] }
 solana-program = "=1.7.8"
 spl-token = { version = "3.1.1", features = ["no-entrypoint"] }