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 16:16:34.096678683 +0800
@@ -39,9 +39,6 @@
 [dev-dependencies.tempfile]
 version = "3"
 
-[dev-dependencies.tokio]
-version = "0.1.7"
-
 [dev-dependencies.tokio-codec]
 version = "0.1.0"