updates getting_started to require factory_girl_rails

I ran into `uninitialized constant FactoryGirl (NameError)` when referencing the documentation, and noted in a [stack overflow](http://stackoverflow.com/questions/22715519/having-trouble-with-rspec-testing-uninitialized-constant-factorygirl-name-erro) the fix. 

Is there actually not normally a need for this, and the require is an edgecase? if so, then i can adjust to note that you may need it instead of saying its required.