Bump futures from 0.1.29 to 0.3.4

Bumps [futures](https://github.com/rust-lang/futures-rs) from 0.1.29 to 0.3.4.
- [Release notes](https://github.com/rust-lang/futures-rs/releases)
- [Changelog](https://github.com/rust-lang/futures-rs/blob/master/CHANGELOG.md)
- [Commits](rust-lang/futures-rs@0.1.29...0.3.4)

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

Author: dependabot-preview[bot]

Cargo.lock

@@ -10,7 +10,7 @@ license = "MIT"
 [dependencies]
 actix-web = "1.0.8"
 serde = "1.0.104"
-futures = "0.1.29"
+futures = "0.3.4"
 rand = "0.7.2"
 actix-web-httpauth = "0.3.2"
 syntect = "3.3.0"