Remove dev dependency on tokio to prevent circular dependency.
--- a/Cargo.toml	1970-01-01 08:00:00.000000000 +0800
+++ b/Cargo.toml	2019-08-15 16:22:31.861095845 +0800
@@ -56,8 +56,5 @@
 [dev-dependencies.num_cpus]
 version = "1.8.0"
 
-[dev-dependencies.tokio]
-version = "0.1.7"
-
 [dev-dependencies.tokio-io-pool]
 version = "0.1.4"