Add ruby 2.4.0 targets to Rakefile.

Rakefile

@@ -107,10 +107,10 @@ task 'gem:native' do
   if RUBY_PLATFORM =~ /darwin/
     FileUtils.touch 'grpc_c.32.ruby'
     FileUtils.touch 'grpc_c.64.ruby'
-    system "rake cross native gem RUBY_CC_VERSION=2.3.0:2.2.2:2.1.5:2.0.0 V=#{verbose} GRPC_CONFIG=#{grpc_config}"
+    system "rake cross native gem RUBY_CC_VERSION=2.4.0:2.3.0:2.2.2:2.1.5:2.0.0 V=#{verbose} GRPC_CONFIG=#{grpc_config}"
   else
     Rake::Task['dlls'].execute
-    docker_for_windows "bundle && rake cross native gem RUBY_CC_VERSION=2.3.0:2.2.2:2.1.5:2.0.0 V=#{verbose} GRPC_CONFIG=#{grpc_config}"
+    docker_for_windows "bundle && rake cross native gem RUBY_CC_VERSION=2.4.0:2.3.0:2.2.2:2.1.5:2.0.0 V=#{verbose} GRPC_CONFIG=#{grpc_config}"
   end
 end