Update logstash.js

[jfstgermain]

Checking if socket.unref exists. In node 0.10 the CleartextStream object returned by tls.connect doesn't have an unref() function

See #18

[rossille]

hi @jfstgermain
First, thank you for this pull request and sorry for the late answer. I'm taking over management of this lib, since the author doesn't have the time to do it anymore.
I had to move the projet to https://github.com/transcovo/bunyan-logstash-tcp and add tests before being able to accept pull requests.
I think that this pull request is not needed anymore since I dropped the support of node 0.x in the 1.0.0 version, but please know that:

• If you still need this, I can publish 1 more version of the 0.x vertion which still supports node 0.x with your change included
• If you want to contribute to the project, I will be accepting pull requests on the new repo https://github.com/transcovo/bunyan-logstash-tcp