Properly generating well known proto files for the macos build.

ruby/Rakefile

diff --git a/ruby/Rakefile b/ruby/Rakefile
index 88a07a6..fa29c31 100644
--- a/ruby/Rakefile
+++ b/ruby/Rakefile
@@ -69,6 +69,7 @@
 
   if RUBY_PLATFORM =~ /darwin/
     task 'gem:native' do
+      system "rake genproto"
       system "rake cross native gem RUBY_CC_VERSION=2.3.0:2.2.2:2.1.5:2.0.0"
     end
   else