Update netty-handler to latest version 4.1.59.Final

[danotorrey]

Update the netty-handler dependency to 4.1.59.Final. This resolves an important CVE as mentioned in #50. I'm not sure why Dependabot did not pick up the later version, so this PR was filed.

[coveralls]

Coverage remained the same at 78.732% when pulling a59c051 on netty-update into a02819a on master.

[danotorrey]

I smoke-tested the UDP and TCP GELF transports, and they seem to work. Also, a review of the Netty change log looks ok as far as I can see.

[danotorrey]

@bernd Shall we cut a release of the gelfclient now that we've updated the Netty version? I don't see any other issues or PRs that we'd want to get it urgently. So, perhaps now makes sense unless we have plans to do it at some point later.

[bernd]

@danotorrey We can do that, yes. 👍 Can you talk to @juju2112 about automating the release process including pushing the artifacts to maven central?

[danotorrey]

@bernd Will do. Thanks.

[danotorrey]

@juju2112 will be looking at doing the gelfclient release sometime in the next weeks.