Bump gelf to 3.0.0
Bump gelf to 3.0.0

Bumps [gelf](https://github.com/Graylog2/gelf-rb) from 2.0.0 to 3.0.0.
- [Commits](https://github.com/Graylog2/gelf-rb/commits)
theodi/panopticon,theodi/panopticon,theodi/panopticon,theodi/panopticon