minor: pin to async-std 1.5.x (#187)

Author: saghm

Cargo.toml

@@ -48,7 +48,7 @@ webpki = "0.21.0"
 webpki-roots = "0.18.0"
 
 [dependencies.async-std]
-version = "1.5.0"
+version = "~1.5.0"
 optional = true
 
 [dependencies.pbkdf2]