Add gem install bundler to installation guide

Created by: pschultz

bundler was missing when I followed this guide. Ubuntu points you to the ruby-bundler, which is based on ruby 1.8 and doesn't work. Installing bundler with gem works.

This PR adds the command to do that to the installation guide.

lib/tasks/gitlab/check.rake

@@ -142,7 +142,7 @@ namespace :gitlab do
         return
       end
-      recipe_content = `curl https://raw.github.com/gitlabhq/gitlab-recipes/master/init.d/gitlab 2>/dev/null`
+      recipe_content = `curl https://raw.github.com/gitlabhq/gitlab-recipes/4-2-stable/init.d/gitlab 2>/dev/null`
       script_content = File.read(script_path)
       if recipe_content == script_content