Ruby build fix mac 3.7.x (#5519)

* Fix gem build for mac

* Update version to 3.7.0

* Change base ruby to 2.5.0

* Revert unnecessary changes

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

@@ -47,7 +47,7 @@ EOF
 
 MAKE="make -j8"
 
-for v in 2.5.0 2.4.0 2.3.0 2.2.2 2.1.6 2.0.0-p645 ; do
+for v in 2.5.1 2.4.0 2.3.0 ; do
   ccache -c
   rake -f "$CROSS_RUBY" cross-ruby VERSION="$v" HOST=x86_64-darwin11 MAKE="$MAKE"
 done

ruby/Rakefile

@@ -76,7 +76,7 @@ else
   if RUBY_PLATFORM =~ /darwin/
     task 'gem:native' do
       system "rake genproto"
-      system "rake cross native gem RUBY_CC_VERSION=2.5.0:2.4.0:2.3.0:2.2.2:2.1.6:2.0.0"
+      system "rake cross native gem RUBY_CC_VERSION=2.5.1:2.4.0:2.3.0"
     end
   else
     task 'gem:native' => [:genproto, 'gem:windows']