Update //third_party/protobuf to version 3.

third_party/protobuf/ruby/google-protobuf.gemspec

+Gem::Specification.new do |s|
+  s.name        = "google-protobuf"
+  s.version     = "3.0.0.alpha.5.0"
+  s.licenses    = ["BSD"]
+  s.summary     = "Protocol Buffers"
+  s.description = "Protocol Buffers are Google's data interchange format."
+  s.homepage    = "https://developers.google.com/protocol-buffers"
+  s.authors     = ["Protobuf Authors"]
+  s.email       = "protobuf@googlegroups.com"
+  s.require_paths = ["lib"]
+  s.files       = `git ls-files -z`.split("\x0").find_all{|f| f =~ /lib\/.+\.rb/}
+  unless RUBY_PLATFORM == "java"
+    s.files     += `git ls-files "*.c" "*.h" extconf.rb Makefile`.split
+    s.extensions= ["ext/google/protobuf_c/extconf.rb"]
+  else
+    s.files     += ["lib/google/protobuf_java.jar"]
+  end
+  s.test_files  = ["tests/basic.rb",
+                  "tests/stress.rb",
+                  "tests/generated_code_test.rb"]
+  s.add_development_dependency "rake-compiler"
+  s.add_development_dependency "test-unit"
+  s.add_development_dependency "rubygems-tasks"
+end