add remove trucks

#40 
This can be refactored a little bit:

```ruby
user = FactoryGirl.create(:user)
truck = FactoryGirl.create(:truck)

sign_in_as(user)

visit food_truck_path(truck)

expect(page).to_not have_button 'Delete Food truck'
```

Same pattern can be applied to some of the other tests here.