Rails/Bootstrap導入

Gemfileを修正
gem 'bootstrap-sass', '~> 3.3.6'
gem 'sass-rails', '~> 5.0'

コマンドを実行
$ bundle install

app/assets/stylesheets/application.css.scss
@import 'bootstrap-sprockets';
@import 'bootstrap';

app/assets/javascripts/application.js
//= require bootstrap-sprockets