message: nightly-examples: fix nightly builds
  This PR fixes nightly builds by opting to use `tokio = "0.2.0-alpha.6"`
  instead of a git dependency.
diff: @@ -15,7 +15,7 @@ edition = "2018"
  tracing = "0.1"
  tracing-subscriber = { version = "0.1", path = "../tracing-subscriber", features = ["json"] }
  tracing-futures = { path = "../tracing-futures", default-features = false, features = ["std-future"] }
  -tokio = { git = "https://github.com/tokio-rs/tokio.git" }
  +futures-preview = { version = "0.3.0-alpha.19", features = ["async-await"] }
  +tokio = "0.2.0-alpha.6"
  tracing-attributes = { path = "../tracing-attributes" }
  -futures-preview = "0.3.0-alpha.18"
  clap = "2.33"
  