Under
development, :test

gem 'rails-controller-testing'

gem 'rspec-rails', '~> 3.8'

gem "better_errors"

gem "binding_of_caller"

under :production

gem 'rails_12factor'

Use the form partial for your edit and new views. Then replace new and edit with <%= render 'form' %>
Add this to the form partial

<%= form_for @wiki do |f| %>
<%= f.label :title %>
<%= f.text_field :title %>

<%= f.label :body %>
<%= f.text_area :body %>

<%= f.submit "Save" %>
<% end %>

app/controllers/wikis_controller.rb

You're using wiki. Changed to wikis. So, it should look like app/views/wikis

t.boolean :private t.references :user, foreign_key: true

Your migration is missing
private:boolean & user:references:index