Rails 4 / Actionmailer / Jqueryが機能しない

前提・実現したいこと

Rails 4にて以下の要領でお問い合わせフォームを作成中です。
https://www.imd-net.com/column/1731/

発生している問題・エラーメッセージ

問い合わせ機能そのものではなく、jqueryが機能しない問題です。
使っているbootstrapのテンプレートの、ドロップダウンが機能しません。
以下ソースコードの通り、ホームと問い合わせページのみのシンプルな構造です。
turbolinksはアプリケーション全体で切ってあります。

該当のソースコード

route.rb

Rails.application.routes.draw do

  get 'contact' => 'inquiry#contact'
  post 'contact/confirm' => 'inquiry#confirm'
  post 'contact/thanks' => 'inquiry#thanks'

  root 'static_pages#home'
  match '/home', to: 'static_pages#home', via: 'get'
end

inquiry_controller.rb

class InquiryController < ApplicationController
  def contact
    # 入力画面を表示
    @inquiry = Inquiry.new
    render :action => 'contact'
  end

  def confirm
    # 入力値のチェック
    @inquiry = Inquiry.new(params[:inquiry])
    if @inquiry.valid?
      # OK。確認画面を表示
      render :action => 'confirm'
    else
      # NG。入力画面を再表示
      render :action => 'index'
    end
  end

  def thanks
    # メール送信
    @inquiry = Inquiry.new(params[:inquiry])
    InquiryMailer.received_email(@inquiry).deliver

    # 完了画面を表示
    render :action => 'thanks'
  end
end

gemfile

source 'https://rubygems.org'


# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.2.6'
# Use sqlite3 as the database for Active Record
gem 'sqlite3'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'
# Use CoffeeScript for .coffee assets and views
gem 'coffee-rails', '~> 4.1.0'
# See https://github.com/rails/execjs#readme for more supported runtimes
gem 'therubyracer', platforms: :ruby

# Use jquery as the JavaScript library
gem 'jquery-rails'
# Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
# gem 'turbolinks'
# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 2.0'
# bundle exec rake doc:rails generates the API under doc/api.
gem 'sdoc', '~> 0.4.0', group: :doc

# Use ActiveModel has_secure_password
# gem 'bcrypt', '~> 3.1.7'

# Use Unicorn as the app server
# gem 'unicorn'

# Use Capistrano for deployment
# gem 'capistrano-rails', group: :development

# gem 'jquery-turbolinks'
gem 'figaro'
gem 'passenger'
gem 'jquery-ui-rails'

group :production do
  gem 'pg'
  gem 'rails_12factor'
end

group :development, :test do
  # Call 'byebug' anywhere in the code to stop execution and get a debugger console
  gem 'byebug'
  gem 'pry-rails'
end

group :development do
  # Access an IRB console on exception pages or by using <%= console %> in views
  gem 'web-console', '~> 2.0'

  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
end

application.js

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, vendor/assets/javascripts,
// or any plugin's vendor/assets/javascripts directory can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file.
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives.
//
//= require jquery
//= require jquery_ujs
//= require_tree .