Added a rake task to automatically download the latest metaschemas

The metaschemas change occasionally. Draft6 is changing all the time
as new features are added. So we can keep on top of this, I've added a
new rake task which will download all of the latest metaschemas and
save them over the existing ones (in the resources directory)

Rakefile

@@ -19,6 +19,43 @@ task :update_common_tests do
   end
 end
 
+desc "Update meta-schemas to the latest version"
+task :update_meta_schemas do
+  puts "Updating meta-schemas..."
+
+  id_mappings = {
+    'http://json-schema.org/draft/schema#' => 'https://raw.githubusercontent.com/json-schema-org/json-schema-spec/master/schema.json'
+  }
+
+  require 'open-uri'
+  require 'thwait'
+
+  download_threads = Dir['resources/*.json'].map do |path|
+    schema_id = File.read(path)[/"\$?id"\s*:\s*"(.*?)"/, 1]
+    schema_uri = id_mappings[schema_id] || schema_id
+
+    Thread.new(schema_uri) do |uri|
+      Thread.current[:uri] = uri
+
+      begin
+        metaschema = URI(uri).read
+
+        File.write(path, metaschema)
+      rescue StandardError
+        false
+      end
+    end
+  end
+
+  ThreadsWait.all_waits(*download_threads) do |t|
+    if t.value
+      puts t[:uri]
+    else
+      STDERR.puts "Failed to update meta-schema #{t[:uri]}"
+    end
+  end
+end
+
 Rake::TestTask.new do |t|
   t.libs << "."
   # disabled warnings because addressable 2.4 has lots of them
@@ -27,4 +64,6 @@ Rake::TestTask.new do |t|
   t.test_files = FileList.new('test/*_test.rb')
 end
 
+task update: [:update_common_tests, :update_meta_schemas]
+
 task :default => :test