* Add ruby 2.6 test

* Fix broken tests

* Fix compatibility test

* Fix bundler version

* Use new docker image

* Fix broken test

* Use new docker image in ruby26 test

* Install ruby for mac

* Add ruby 2.3, 2.4 and 2.5 test for linux.

* Update kokoro files

* Add back commented gc test

* Fix gc_test failure

* Remove unused code

* Update ruby 2.5.0 to 2.5.1

* Update ruby 2.3 to 2.3.8

* Remove useless comment