Enable "async-await" for futures-preview (select!)

[ci skip]

Cargo.toml

@@ -26,7 +26,6 @@ docs = []
 async-task = "1.0.0"
 cfg-if = "0.1.9"
 crossbeam-channel = "0.3.9"
-futures-preview = "0.3.0-alpha.17"
 futures-timer = "0.3.0"
 lazy_static = "1.3.0"
 log = { version = "0.4.8", features = ["kv_unstable"] }
@@ -37,6 +36,10 @@ num_cpus = "1.10.0"
 pin-utils = "0.1.0-alpha.4"
 slab = "0.4.2"
 
+[dependencies.futures-preview]
+version = "0.3.0-alpha.17"
+features = ["async-await", "nightly"]
+
 [dev-dependencies]
 femme = "1.1.0"
 tempdir = "0.3.7"