Move "Install Packages" before "Manage Installed Versions"

This order seems more logical to me and matches the other distribution installation instruction pages - http://docs.mongodb.org/manual/tutorial/install-mongodb-on-debian/ and http://docs.mongodb.org/manual/tutorial/install-mongodb-on-red-hat-centos-or-fedora-linux/

Signed-off-by: Sam Kleinman <samk@10gen.com>