Update Xcode version needed to run Kokoro tests and replace deprecated rubygem options (#6890)

* Update Xcode version needed to run Kokoro tests

* Replace deprecated --no-ri --no-rdoc rubygem options with --no-document

kokoro/macos/prepare_build_macos_rc

diff --git a/kokoro/macos/prepare_build_macos_rc b/kokoro/macos/prepare_build_macos_rc
index 49fb99a..acb2aba 100755
--- a/kokoro/macos/prepare_build_macos_rc
+++ b/kokoro/macos/prepare_build_macos_rc
@@ -5,10 +5,11 @@
 ##
 # Select Xcode version
 
-# Remember to udpate the Xcode version when xcode_9.4.1 is not available.
-# If xcode is not available, it will probaly encounter the failure for 
-# "autom4te: need GNU m4 1.4 or later: /usr/bin/m4""
-export DEVELOPER_DIR=/Applications/Xcode_9.4.1.app/Contents/Developer
+# Remember to udpate the Xcode version when Xcode_11.0.app is not available.
+# If xcode is not available, it will probaly encounter the failure for
+# "autom4te: need GNU m4 1.4 or later: /usr/bin/m4"
+# go/kokoro/userdocs/macos/selecting_xcode.md for more information.
+export DEVELOPER_DIR=/Applications/Xcode_11.0.app/Contents/Developer
 
 ##
 # Select C/C++ compilers

kokoro/release/ruby/macos/ruby/ruby_build_environment.sh

diff --git a/kokoro/release/ruby/macos/ruby/ruby_build_environment.sh b/kokoro/release/ruby/macos/ruby/ruby_build_environment.sh
index d42a0a8..5776e3c 100755
--- a/kokoro/release/ruby/macos/ruby/ruby_build_environment.sh
+++ b/kokoro/release/ruby/macos/ruby/ruby_build_environment.sh
@@ -7,8 +7,8 @@
 set -e  # rvm commands are very verbose
 time rvm install 2.5.0
 rvm use 2.5.0 --default
-gem install rake-compiler --no-ri --no-doc
-gem install bundler --no-ri --no-doc
+gem install rake-compiler --no-document
+gem install bundler --no-document
 rvm osx-ssl-certs status all
 rvm osx-ssl-certs update all
 set -ex
@@ -54,4 +54,3 @@
 
 sed 's/x86_64-darwin-11/universal-darwin/' ~/.rake-compiler/config.yml > "$CROSS_RUBY"
 mv "$CROSS_RUBY" ~/.rake-compiler/config.yml
-