Update pin-project to 0.4 (#111)

Author: taiki-e

Cargo.toml

@@ -21,7 +21,7 @@ futures-preview = "0.3.0-alpha.18"
 runtime-attributes = { path = "runtime-attributes", version = "0.3.0-alpha.6", default-features = false }
 runtime-raw = { path = "runtime-raw", version = "0.3.0-alpha.5" }
 runtime-native = { path = "runtime-native", version = "0.3.0-alpha.6", optional = true }
-pin-project = "0.4.0-beta.1"
+pin-project = "0.4"
 
 [dev-dependencies]
 failure = "0.1.5"