Add ruby 2.3, 2.4 and 2.5 test for linux. (#5256)

* 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
diff --git a/tests.sh b/tests.sh
index d5ac6e5..2d3d1d4 100755
--- a/tests.sh
+++ b/tests.sh
@@ -254,7 +254,7 @@
 }
 build_ruby23() {
   internal_build_cpp  # For conformance tests.
-  cd ruby && bash travis-test.sh ruby-2.3 && cd ..
+  cd ruby && bash travis-test.sh ruby-2.3.8 && cd ..
 }
 build_ruby24() {
   internal_build_cpp  # For conformance tests.
@@ -262,7 +262,7 @@
 }
 build_ruby25() {
   internal_build_cpp  # For conformance tests.
-  cd ruby && bash travis-test.sh ruby-2.5.0 && cd ..
+  cd ruby && bash travis-test.sh ruby-2.5.1 && cd ..
 }
 build_ruby_all() {
   build_ruby23