[1.2.X] Fixed #14863 - Add contrib.messages to list of default INSTALLED_APPS in tutorial.

Backport of r14870 (without staticfiles) from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14871 bcc190cf-cafb-0310-a4f2-bffc1f526a37