Moved pubsub demo_demo to examples directory of ruby

so that grpc does not depends on googleauth gem.

This project (pubsub demo) is introduced at [this
commit](d5d00d5c8f (diff-c495642bb64c09cae54f3ccff463bd51))
as demo project for grpc with ruby.
This demo is awesome. But the position of this project directory seems
to be not suitable for demo app. This project added unnecessary gem dependency about googleauth to grpc ruby gem.

examples/ruby/grpc-demo.gemspec

@@ -18,6 +18,7 @@ Gem::Specification.new do |s|
   s.platform      = Gem::Platform::RUBY
 
   s.add_dependency 'grpc', '~> 1.0'
+  s.add_dependency 'googleauth', '>= 0.5.1', '< 0.7'
 
   s.add_development_dependency 'bundler', '~> 1.7'
 end