Remove unused features to reduce dependency.
--- a/Cargo.toml	1970-01-01 08:00:00.000000000 +0800
+++ b/Cargo.toml	2019-05-28 13:46:20.360693993 +0800
@@ -31,7 +31,6 @@
 
 [dev-dependencies.loom]
 version = "0.1.1"
-features = ["futures"]
 
 [dev-dependencies.tokio]
 version = "0.1.15"