Updated to ruby-2.5

[bayars]

I updated and tested for stability. It's working with ruby-2.5 right now. I couldn't update rake reason for some functions inside the code. I will update again with rake 13.0.1. Also, I find some bugs with rubocop. I will open other PR's for those.

The ruby-gelf continue using many companies. The new version must publish on the gem repositories.

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