Add ruby2.3,2.4.2.5 test on Mac and disable ruby2.1,2.2 test on Linux (#5250)

kokoro/macos/ruby23/build.sh

+#!/bin/bash
+#
+# Build file to set up and run tests
+
+# Change to repo root
+cd $(dirname $0)/../../..
+
+# Prepare worker environment to run tests
+source kokoro/macos/prepare_build_macos_rc
+
+./tests.sh ruby23

kokoro/macos/ruby23/continuous.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/macos/ruby23/build.sh"
+timeout_mins: 1440

kokoro/macos/ruby23/presubmit.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/macos/ruby23/build.sh"
+timeout_mins: 1440

kokoro/macos/ruby24/build.sh

+#!/bin/bash
+#
+# Build file to set up and run tests
+
+# Change to repo root
+cd $(dirname $0)/../../..
+
+# Prepare worker environment to run tests
+source kokoro/macos/prepare_build_macos_rc
+
+./tests.sh ruby24

kokoro/macos/ruby24/continuous.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/macos/ruby24/build.sh"
+timeout_mins: 1440

kokoro/macos/ruby24/presubmit.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/macos/ruby24/build.sh"
+timeout_mins: 1440

kokoro/macos/ruby25/build.sh

+#!/bin/bash
+#
+# Build file to set up and run tests
+
+# Change to repo root
+cd $(dirname $0)/../../..
+
+# Prepare worker environment to run tests
+source kokoro/macos/prepare_build_macos_rc
+
+./tests.sh ruby25

kokoro/macos/ruby25/continuous.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/macos/ruby25/build.sh"
+timeout_mins: 1440

kokoro/macos/ruby25/presubmit.cfg

+# Config file for running tests in Kokoro
+
+# Location of the build script in repository
+build_file: "protobuf/kokoro/macos/ruby25/build.sh"
+timeout_mins: 1440

tests.sh

@@ -265,8 +265,6 @@ build_ruby25() {
   cd ruby && bash travis-test.sh ruby-2.5.0 && cd ..
 }
 build_ruby_all() {
-  build_ruby21
-  build_ruby22
   build_ruby23
   build_ruby24
   build_ruby25
@@ -575,6 +573,9 @@ Usage: $0 { cpp |
             python_compatibility |
             ruby21 |
             ruby22 |
+            ruby23 |
+            ruby24 |
+            ruby25 |
             jruby |
             ruby_all |
             php5.5   |