Update tutorial regarding mongoid, similar to rails3-mongoid-omniauth

[MarkDBlackwell]

For whoever must use MongoDB versions earlier than 2.2.0, please update the tutorial section 'Installing MongoDB') similar to your comment and pull request for Rails Example App rails3-mongoid-omniauth.

On the Debian GNU/Linux operating system, as of March 2013, the latest stable version is MongoDB 2.0.0. With MongoDB 2.0.0, the Mongoid gem must be version 3.0.x. See the "Mongoid installation instructions":http://mongoid.org/en/mongoid/docs/installation.html#installation. Change your Gemfile to use an earlier Mongoid version:
+
+<pre>
+gem 'mongoid', '~> 3.0.1'
+</pre>