Update Bootstrap documentation (4.0.0)

Author: Thibaut

assets/javascripts/templates/pages/about_tmpl.coffee

@@ -117,7 +117,7 @@ credits = [
     'https://raw.githubusercontent.com/petkaantonov/bluebird/master/LICENSE'
   ], [
     'Bootstrap',
-    '2011-2017 Twitter, Inc.<br>2011-2017 The Bootstrap Authors',
+    '2011-2018 Twitter, Inc.<br>2011-2018 The Bootstrap Authors',
     'CC BY',
     'https://creativecommons.org/licenses/by/3.0/'
   ], [

lib/docs/filters/bootstrap/clean_html_v4.rb

@@ -17,6 +17,8 @@ def call
         end
 
         css('.bd-example', '.responsive-utilities-test').each do |node|
+          next unless node.previous_element
+
           if node.previous_element['class'].try(:include?, 'bd-example')
             node.remove
           else

lib/docs/scrapers/bootstrap.rb

@@ -9,14 +9,14 @@ class Bootstrap < UrlScraper
     options[:trailing_slash] = true
 
     options[:attribution] = <<-HTML
-      &copy; 2011&ndash;2017 Twitter, Inc.<br>
-      &copy; 2011&ndash;2017 The Bootstrap Authors<br>
+      &copy; 2011&ndash;2018 Twitter, Inc.<br>
+      &copy; 2011&ndash;2018 The Bootstrap Authors<br>
       Code licensed under the MIT License.<br>
       Documentation licensed under the Creative Commons Attribution License v3.0.
     HTML
 
     version '4' do
-      self.release = '4.0.0-beta.3'
+      self.release = '4.0.0'
       self.base_url = 'https://getbootstrap.com/docs/4.0/'
       self.root_path = 'getting-started/introduction/'