* Remove ruby21 and ruby22 tests

* Update test badge

* 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

These changes require building new docker images and without new images
our kokoro tests are all failing.

* Fixes https://github.com/google/protobuf/issues/5031
* Fixes https://github.com/google/protobuf/issues/5037

Changes:
1. Remove stuff no longer needed. Lots of the heavy lifting were there
because we were running our own jenkins cluster and had to manage all
the test logs ourselves. Now they are useless.
2. Change "-j2" to "-j4" to speed up the test a little bit. Kokoro
machines have 4 logic CPUs according to their spec.

* upgrade dotnet SDK to 2.1.3 on kokoro linux

1. Added C++ tests under the release docker image.
2. Cleaned up tests.sh and removed traivis stuff.

1. Added C++ tests under the release docker image.
2. Fixed a compile issue with the docker image.
3. Cleaned up tests.sh and removed traivis stuff.

Their job configs are being deleted in cl/190526138

Add googletest as a submodule in third_party/googletest.

This will allow SourceLink as per #4179, and mean that we can use C#
7.0 language features in the library (but not in generated code).
This does not affect which platforms we're *targeting*, so end users
won't see any difference.

It would be nice to update to 2.1.4, but AppVeyor's "Visual Studio
2017" environment is only 2.0.3.