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 16:27:19.881461880 +0800
@@ -34,8 +34,5 @@
 [dev-dependencies.rand]
 version = "0.6"
 
-[dev-dependencies.tokio]
-version = "0.1.7"
-
 [dev-dependencies.tokio-mock-task]
 version = "0.1.0"