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.

app/models/repository.rb

@@ -153,14 +153,14 @@ class Repository
     # Build file path
     file_name = self.path_with_namespace.gsub("/","_") + "-" + commit.id.to_s + ".tar.gz"
     storage_path = Rails.root.join("tmp", "repositories")
-    file_path = File.join(storage_path, file_name)
+    file_path = File.join(storage_path, self.path_with_namespace, file_name)
     # Put files into a directory before archiving
     prefix = self.path_with_namespace + "/"
     # Create file if not exists
     unless File.exists?(file_path)
-      FileUtils.mkdir_p storage_path
+      FileUtils.mkdir_p File.dirname(file_path)
       file = self.repo.archive_to_file(ref, prefix,  file_path)
     end