Fix YARD docs
Change the members to be a frozen set

* I benchmarked the difference between Array#include? and
  Set#include? and the latter was faster even for small sets of
  objects. The performance was outstanding for larger sets.

  It makes sense since Array#include? is O(n) and Set#include? is
  O(1), yet I thought I has read somewhere that Array#include? was
  faster when "n" was small, but that does not seem to be the case,
  at least under ruby 1.9.3.