update ci rubies (#774)

.kokoro/build.sh

@@ -13,23 +13,21 @@ env | grep KOKORO
 
 cd github/google-api-ruby-client/
 
-# Print out Ruby version
-ruby --version
+versions=($RUBY_VERSIONS)
 
 # Temporary workaround for a known bundler+docker issue:
 # https://github.com/bundler/bundler/issues/6154
 export BUNDLE_GEMFILE=
 
-RUBY_VERSIONS=("2.3.8" "2.4.5" "2.5.3" "2.6.1")
-
 # Capture failures
 EXIT_STATUS=0 # everything passed
 function set_failed_status {
     EXIT_STATUS=1
 }
 
-for version in "${RUBY_VERSIONS[@]}"; do
+for version in "${versions[@]}"; do
     rbenv global "$version"
+    echo "Using Ruby $version"
     (bundle update && bundle exec rake) || set_failed_status
 done