Fix incorrect example code for 'builds' in README

[george-carlin]

No description provided.

[Startouf]

The include was definitely missing but then you removed the alignment of the is_a?which feels less clean now.

Actually instead of the if else, although I usually don't like that, the builder is probable the only place where using a case syntax feels nicer

builds do |model, options|
  case model
  when Post; PostCell
  when Comment; CommentCell
  end
end

[apotonick]

Agreeing with @Startouf - in builder, I'm also OK with a case statement, it looks better.

[george-carlin]

fixed and rebased ;)