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.

doc/install/installation.md

@@ -237,6 +237,7 @@ Make sure to update username/password in config/database.yml.
     cd /home/gitlab/gitlab
+    sudo gem install bundler
     sudo gem install charlock_holmes --version '0.6.9'
     # For MySQL (note, the option says "without")