Disable draper stubbing for now

Mocha::StubbingError: stubbing non-existent method:
#<Draper::HelperProxy:0xe181a38>.params
    /home/travis/build/alphagov/whitehall/test/test_helper.rb:187:in
`stubs_helper_method'

/home/travis/build/alphagov/whitehall/test/unit/presenters/api/page_pres
enter_test.rb:5:in `block in <class:PagePresenterTest>'