gem: add Ruby 2.6 support

.travis.yml

@@ -2,6 +2,7 @@
 sudo: false
 language: ruby
 rvm:
+  - 2.6
   - 2.5
   - 2.4
   - 2.3

CHANGELOG.md

@@ -8,7 +8,7 @@ IMPROVEMENTS:
 - Better protocol compliance by returning `badauth` in response body on HTTP 401 errors
 - Better code maintainability by refactorings
 - Update dependencies, mainly `rack` to new major version 2
-- Add Ruby 2.5 support
+- Add Ruby 2.5 and Ruby 2.6 support
 - Add experimental [OpenTracing](http://opentracing.io/) support with [CNCF Jaeger](https://github.com/jaegertracing/jaeger)
 - Support host offlining by deleting the associated DNS records
 - Add textfile reporter to write Graphite-style metrics (also compatible with [Prometheus](https://prometheus.io/)) into a file