Update //third_party/protobuf to version 3.

third_party/protobuf/ruby/README.md

+This directory contains the Ruby extension that implements Protocol Buffers
+functionality in Ruby.
+
+The Ruby extension makes use of generated Ruby code that defines message and
+enum types in a Ruby DSL. You may write definitions in this DSL directly, but
+we recommend using protoc's Ruby generation support with .proto files. The
+build process in this directory only installs the extension; you need to
+install protoc as well to have Ruby code generation functionality.
+
+Installation from Gem
+---------------------
+
+When we release a version of Protocol Buffers, we will upload a Gem to
+[RubyGems](https://www.rubygems.org/). To use this pre-packaged gem, simply
+install it as you would any other gem:
+
+    $ gem install [--prerelease] google-protobuf
+
+The `--pre` flag is necessary if we have not yet made a non-alpha/beta release
+of the Ruby extension; it allows `gem` to consider these "pre-release"
+alpha/beta versions.
+
+Once the gem is installed, you may or may not need `protoc`. If you write your
+message type descriptions directly in the Ruby DSL, you do not need it.
+However, if you wish to generate the Ruby DSL from a `.proto` file, you will
+also want to install Protocol Buffers itself, as described in this repository's
+main `README` file. The version of `protoc` included in the latest release
+supports the `--ruby_out` option to generate Ruby code.
+
+A simple example of using the Ruby extension follows. More extensive
+documentation may be found in the RubyDoc comments (`call-seq` tags) in the
+source, and we plan to release separate, more detailed, documentation at a
+later date.
+
+    require 'google/protobuf'
+
+    # generated from my_proto_types.proto with protoc:
+    #  $ protoc --ruby_out=. my_proto_types.proto
+    require 'my_proto_types'
+
+    mymessage = MyTestMessage.new(:field1 => 42, :field2 => ["a", "b", "c"])
+    mymessage.field1 = 43
+    mymessage.field2.push("d")
+    mymessage.field3 = SubMessage.new(:foo => 100)
+
+    encoded_data = MyTestMessage.encode(mymessage)
+    decoded = MyTestMessage.decode(encoded_data)
+    assert decoded == mymessage
+
+    puts "JSON:"
+    puts MyTestMessage.encode_json(mymessage)
+
+Installation from Source (Building Gem)
+---------------------------------------
+
+To build this Ruby extension, you will need:
+
+* Rake
+* Bundler
+* Ruby development headers
+* a C compiler
+
+To Build the JRuby extension, you will need:
+
+* Maven
+* The latest version of the protobuf java library (see ../java/README.md)
+* Install JRuby via rbenv or RVM
+
+First switch to the desired platform with rbenv or RVM.
+
+Then install the required Ruby gems:
+
+    $ gem install bundler
+    $ bundle
+
+Then build the Gem:
+
+    $ rake
+    $ rake clobber_package gem
+    $ gem install `ls pkg/google-protobuf-*.gem`
+
+To run the specs:
+
+    $ rake test
+
+This gem includes the upb parsing and serialization library as a single-file
+amalgamation. It is up-to-date with upb git commit
+`535bc2fe2f2b467f59347ffc9449e11e47791257`.
+
+Version Number Scheme
+---------------------
+
+We are using a version number scheme that is a hybrid of Protocol Buffers'
+overall version number and some Ruby-specific rules. Gem does not allow
+re-uploads of a gem with the same version number, so we add a sequence number
+("upload version") to the version. We also format alphabetical tags (alpha,
+pre, ...) slightly differently, and we avoid hyphens. In more detail:
+
+* First, we determine the prefix: a Protocol Buffers version "3.0.0-alpha-2"
+  becomes "3.0.0.alpha.2". When we release 3.0.0, this prefix will be simply
+  "3.0.0".
+* We then append the upload version: "3.0.0.alpha.2.0" or "3.0.0.0". If we need
+  to upload a new version of the gem to fix an issue, the version becomes
+  "3.0.0.alpha.2.1" or "3.0.0.1".
+* If we are working on a prerelease version, we append a prerelease tag:
+  "3.0.0.alpha.3.0.pre". The prerelease tag comes at the end so that when
+  version numbers are sorted, any prerelease builds are ordered between the
+  prior version and current version.
+
+These rules are designed to work with the sorting rules for
+[Gem::Version](http://ruby-doc.org/stdlib-2.0/libdoc/rubygems/rdoc/Gem/Version.html):
+release numbers should sort in actual release order.