Update tracing-subscriber requirement from 0.2.12 to 0.3.0 (#360)

Updates the requirements on [tracing-subscriber](https://github.com/tokio-rs/tracing) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/tracing/releases)
- [Commits](tokio-rs/tracing@tracing-subscriber-0.2.12...tracing-subscriber-0.3.0)

---
updated-dependencies:
- dependency-name: tracing-subscriber
  dependency-type: direct:production
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Stephen M. Coakley <[email protected]>

Cargo.toml

@@ -94,7 +94,7 @@ static_assertions = "1.1"
 structopt = "0.3"
 tempfile = "3.1"
 test-case = "1.1"
-tracing-subscriber = "0.2.12"
+tracing-subscriber = ">=0.2.12, <0.4.0"
 serde_json = "1"
 
 [dev-dependencies.testserver]