Update log requirement from 0.3 to 0.4

Updates the requirements on [log](https://github.com/rust-lang/log) to permit the latest version.
- [Release notes](https://github.com/rust-lang/log/releases)
- [Changelog](https://github.com/rust-lang/log/blob/master/CHANGELOG.md)
- [Commits](rust-lang/log@env_logger-0.3.3...0.4.8)

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

Cargo.toml

@@ -24,4 +24,4 @@ unstable = ["lints", "nightly"] # for building with travis-cargo
 libc = "0.2"
 librsync-sys = { version = "0.1", path = "librsync-sys" }
 clippy = { version = "< 1", optional = true }
-log = { version = "0.3", optional = true }
+log = { version = "0.4", optional = true }