Upgrade tracing-subscriber to fix RUSTSEC advisory

[seanpianka]

tracing-subscriber 0.3.0 was released 3 days ago in order to fix RUSTSEC-2020-0159. As aws-lambda-rust-runtime has a 0.2 dependency, it would be best if this dependency was updated to 0.3 to avoid usage of chrono and support new tracing-* releases.

[coltonweaver]

Closing this after merging the related PR.