Remove unused features to reduce dependency.
--- a/Cargo.toml	1970-01-01 08:00:00.000000000 +0800
+++ b/Cargo.toml	2019-08-14 17:54:22.193406174 +0800
@@ -94,7 +94,6 @@
 
 [dev-dependencies.flate2]
 version = "1"
-features = ["tokio"]
 
 [dev-dependencies.futures-cpupool]
 version = "0.1"