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 16:20:14.888936007 +0800
@@ -51,6 +51,3 @@
 version = "0.1.1"
 [dev-dependencies.tempfile]
 version = "3"
-
-[dev-dependencies.tokio]
-version = "0.1.6"