Update futures-timer requirement from 0.2.1 to 0.3.0 (#76)

Updates the requirements on [futures-timer](https://github.com/alexcrichton/futures-timer) to permit the latest version.
- [Release notes](https://github.com/alexcrichton/futures-timer/releases)
- [Changelog](https://github.com/rustasync/futures-timer/blob/master/CHANGELOG.md)
- [Commits](async-rs/futures-timer@0.2.1...0.3.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

runtime-native/Cargo.toml

@@ -21,7 +21,7 @@ async-datagram = "3.0.0"
 juliex = "0.3.0-alpha.6"
 lazy_static = "1.3.0"
 romio = "0.3.0-alpha.9"
-futures-timer = "0.2.1"
+futures-timer = "0.3.0"
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 futures01 = { package = "futures", version = "0.1" }