Remove use of deprecated ActiveSupport::Memoizable

[lewinski]

Resubmitting PR#3 because github got confused.

The ActiveSupport::Memoizable module is deprecated in ActiveSupport 3.2, so this just cleans up the warnings that running the doc generator emits during load.

It didn't seem like it was actually being used in ApiGuides::Document.

For the memoization in ApiGuides::Generator, the output of that pipeline shouldn't be nil, so the memoization there is really cheap and dirty.