Bump futures-preview from 0.3.0-alpha.18 to 0.3.0-alpha.19

Bumps [futures-preview](https://github.com/rust-lang-nursery/futures-rs) from 0.3.0-alpha.18 to 0.3.0-alpha.19.
- [Release notes](https://github.com/rust-lang-nursery/futures-rs/releases)
- [Changelog](https://github.com/rust-lang-nursery/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.3.0-alpha.18...0.3.0-alpha.19)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

Cargo.lock

@@ -7,7 +7,7 @@ edition = '2018'
 [dependencies]
 env_logger = "0.6"
 failure = "0.1"
-futures-preview = { version = "0.3.0-alpha.18", features = ["compat"] }
+futures-preview = { version = "0.3.0-alpha.19", features = ["compat"] }
 log = "0.4"
 serde = "1"
 serde_derive = "1"