Spec failures in Rails 4.2

[arthurchui]

There are 3 broken specs in Rails 4.2. Fix them if they are not too difficult; otherwise we should officially discontinue to support Rails 4.

# Update jit_preloader.gemspec
spec.add_dependency "activerecord", ">= 4.2"
spec.add_development_dependency "sqlite3", "< 1.4"

> bundle exec rspec spec

  1) JitPreloader::Preloader for single table inheritance when preloading an aggregate of a child model through its base model can handle queries
     Failure/Error: expect(contact_books.first.employees_count).to eq 4

       expected: 4
            got: 0

       (compared using ==)
     # ./spec/lib/jit_preloader/preloader_spec.rb:75:in `block (4 levels) in <top (required)>'

  2) JitPreloader::Preloader for single table inheritance when preloading an aggregate of a nested child model through another child model can handle queries
     Failure/Error: expect(contact_books.first.company_employees_count).to eq 2

       expected: 2
            got: 0

       (compared using ==)
     # ./spec/lib/jit_preloader/preloader_spec.rb:91:in `block (4 levels) in <top (required)>'

  3) JitPreloader::Preloader for single table inheritance when preloading an aggregate of a nested child model through a many-to-many relationship with another child model can handle queries
     Failure/Error: expect(contact_books.first.children_count).to eq 4

       expected: 4
            got: 0

       (compared using ==)
     # ./spec/lib/jit_preloader/preloader_spec.rb:105:in `block (4 levels) in <top (required)>'