Added gettext requirement for i18n

The edx-platform i18n rake tasks complain if `gettext` isn't installed with this error message:

        Cannot locate GNU gettext utilities, which are required by django for internationalization.
        (see https://docs.djangoproject.com/en/dev/topics/i18n/translation/#message-files)
        Try downloading them from http://www.gnu.org/software/gettext/