Test against Ruby 2.6 and fix Travis (Netflix#3)

* Test against Ruby 2.6

* Enforce bundler 1.17.3

Author: krzysiek1507

.travis.yml

@@ -4,5 +4,12 @@ rvm:
   - 2.3.6
   - 2.4.3
   - 2.5.0
+  - 2.6
+
+before_install:
+  - "travis_retry gem update --system 2.7.9"
+  - "travis_retry gem install bundler -v '1.17.3'"
+install: BUNDLER_VERSION=1.17.3 bundle install --path=vendor/bundle --retry=3 --jobs=3
+
 script:
   - bundle exec rspec