title: File to import not found or unreadable: bootstrap-sprockets on rails 5.2.0
body: <p>I have following problem.
  "File to import not found or unreadable: bootstrap-sprockets."</p>
  
  <p>Following gems I have installed
  gem 'bootstrap'
  gem 'bootstrap-sass', '~> 3.3.7'
  gem 'sass-rails', '>= 3.2'</p>
  
  <p>File: Application name change from css to scss.
  In the application.scss file I add the lines</p>
  
  <p>@import "bootstrap-sprockets";
  @import "bootstrap";  </p>
  
  <p>I bundle install and afterwards restart the server. 
  But still the same error message. It will be great if somebody had the same issue and found a solution, all written solutions on stackoverflow doesn't work.</p>