Update socket2 requirement from 0.3.16 to 0.4.0 (#382)

Updates the requirements on [socket2](https://github.com/rust-lang/socket2) to permit the latest version.
- [Release notes](https://github.com/rust-lang/socket2/releases)
- [Changelog](https://github.com/rust-lang/socket2/blob/master/CHANGELOG.md)
- [Commits](rust-lang/socket2@v0.3.16...v0.4.0)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Cargo.toml

@@ -18,7 +18,7 @@ appveyor = { repository = "alexcrichton/curl-rust" }
 [dependencies]
 libc = "0.2.42"
 curl-sys = { path = "curl-sys", version = "0.4.41", default-features = false }
-socket2 = "0.3.16"
+socket2 = "0.4.0"
 
 # Unix platforms use OpenSSL for now to provide SSL functionality
 [target.'cfg(all(unix, not(target_os = "macos")))'.dependencies]