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 16:32:06.281824879 +0800
@@ -40,6 +40,3 @@
 [dev-dependencies.env_logger]
 version = "0.5"
 default-features = false
-
-[dev-dependencies.tokio]
-version = "0.1.13"