upgrade to futures 0.3 (#9)

Signed-off-by: Yoshua Wuyts <[email protected]>

Author: yoshuawuyts

Cargo.toml

@@ -12,8 +12,8 @@ readme = "README.md"
 edition = "2018"
 
 [dependencies]
-futures-core-preview = "0.3.0-alpha.18"
+futures-core = "0.3.0"
 pin-utils = "0.1.0-alpha.4"
 
 [dev-dependencies]
-futures-preview = "0.3.0-alpha.18"
+futures = "0.3.0"