Upgrade tracing-subscriber to 0.3.1 (#55)

Author: JohnTitor

Cargo.lock

@@ -16,4 +16,4 @@ tracing = "0.1.29"
 [dev-dependencies]
 conduit-test = { version = "0.9.0-alpha.5", path = "../conduit-test" }
 lazy_static = "1.4.0"
-tracing-subscriber = "0.2.25"
+tracing-subscriber = { version = "0.3.1", features = ["env-filter"] }