Install: Fixes "Don't know how to build task 'cms:update_templates'"

Remove the `update_templates command from the installation script, this
command was removed by
https://github.com/edx/edx-platform/commit/3722685e1a185ff4a0a085bb380ed793cf6d1e59
Merge pull request #431 from antoviaque/updatetemplate-notfound

Install: Fixes "Don't know how to build task 'cms:update_templates'"