read_activity.js

var request = require('request')
  , gunzip = require('zlib').createGunzip()
  , json = new (require('jstream'))();

request('http://data.githubarchive.org/2012-03-11-12.json.gz')
  .pipe(gunzip)
  .pipe(json)
    .on('data', function(obj) {
      console.log(obj);
    });

read_activity.rb

require 'open-uri'
require 'zlib'
require 'yajl'

gz = open('http://data.githubarchive.org/2012-03-11-12.json.gz')
js = Zlib::GzipReader.new(gz).read

Yajl::Parser.parse(js) do |event|
  print event
end

does jstream have the same semantics as JSONStream?

Not sure. I haven't used jsonstream or jstream before. I just looked around for the first thing that looked promising and depended on @dscape's clarinet. But it seems to be a valid stream.