added model specs and adjusted Travis setup

### question 1

the migrations I originally created for the project required `Element` to have a `question_sheet_id`, but a recent version of the  [`sp` schema](https://github.com/CruGlobal/sp/blob/cd835f4303547c890ca0a665bd957c6790aecbe6/db/schema.rb) shows an inconsistent implementation of this: IE, 
- `pr_elements` = does not have a `question_sheet_id` column
- `si_elements` = `question_sheet_id     null: false`
- `sp_elements` = does not have a `question_sheet_id` column 

I'd suggestion either option, 

1) change qe engine migrations to drop the `Element.question_sheet_id .. null: false`.

2) change the production schema to `Element.question_sheet_id .. null: false`.

### question 2 

Depending on answer to previous question, should `Element` have an AR::Relationship linking to `QuestionSheet`? EG,  

```ruby 
class Element < ActiveRecord::Base
  belongs_to :question_sheet
end
```