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

@@ -147,7 +147,7 @@ Fix the directory permissions for the repositories:
     # Make sure the repositories dir is owned by git and it stays that way
     sudo chmod -R ug+rwX,o-rwx /home/git/repositories/
     sudo chown -R git:git /home/git/repositories/
-    find /home/git/repositories -type d -print0 | sudo xargs -0 chmod g+s
+    sudo -u git -H find /home/git/repositories -type d -print0 | sudo xargs -0 chmod g+s
 ## Add domains to list to the list of known hosts