Fixed 'rake preview on fails vanilla install' bug

Rakefile

@@ -182,9 +182,7 @@ task :stop_serve do
 end
 
 desc "preview the site in a web browser"
-multitask :preview => [:generate, :start_serve, :watch] do
+task :preview => [:generate, :start_serve, :watch]
-end
-
 
 desc "Build an XML sitemap of all html files."
 task :sitemap do
@@ -217,4 +215,4 @@ task :sitemap do
     sitemap.puts %Q{</urlset>}
     puts "Created #{site}/sitemap.xml"
   end
-end
+end