Add exam counter

* Gemfile: Enforced v2.1.0.0 because latest version broke the page layout, possibly because Bootstrap 3 no longer separates a responsive template.

* app/models/quiz.rb: Exams already had belongs_to :quiz. Needed has_many for User method count_completed_exams_by_quiz.

* app/models/user.rb: Count logic here.

* app/views/families/show.html.haml: Only displays if user logged in and count > 0.