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 16:33:07.665891615 +0800
@@ -26,5 +26,3 @@
 
 [dependencies.futures]
 version = "0.1.19"
-[dev-dependencies.tokio]
-version = "0.1.17"