Update to futures-preview 0.3.0-alpha.16 (#230)

example-service/src/lib.rs

@@ -4,12 +4,7 @@
 // license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
-#![feature(
-    arbitrary_self_types,
-    await_macro,
-    async_await,
-    proc_macro_hygiene
-)]
+#![feature(arbitrary_self_types, async_await, proc_macro_hygiene)]
 
 // This is the service definition. It looks a lot like a trait definition.
 // It defines one RPC, hello, which takes one arg, name, and returns a String.