Remove dev dependency on tokio to prevent circular dependency.
--- a/Cargo.toml	2019-08-15 16:09:12.420168154 +0800
+++ b/Cargo.toml	2019-08-15 16:09:00.092154087 +0800
@@ -32,8 +32,5 @@
 [dev-dependencies.loom]
 version = "0.1.1"
 
-[dev-dependencies.tokio]
-version = "0.1.15"
-
 [dev-dependencies.tokio-mock-task]
 version = "0.1.1"