speed up fixtures by not loading all their classes

this took our time for a test with `tixtures :all`  from 28s to 20s

most of the time, only the tables are needed and `fixtures :all` is just a developer saying "I don't know",
so let's save the complexity and overhead!

people that rely on that can do their own requires or use eager loading in test.rb

@rafaelfranca @tenderlove 


:+1: We already look up model classes by constantizing anyway.

As far as I can tell, requiring them here predates that, so we're doing duplicate work now.