versions: restore original constant

Before:

  f1 = Formula.factory('tree')
  f1.versions
  f2 = Formula.factory('tree')
  f1.class == f2.class # => false

After:

  f1 = Formula.factory('tree')
  f1.versions
  f2 = Formula.factory('tree')
  f1.class == f2.class # => true