fix promotion logo file name
Merge pull request #61 from zendesk/likun/fix_promotion_logo

fix promotion logo file name
bump v1.18.1
use lowercase locale ids except for default locale 'en-US'
Merge pull request #63 from zendesk/likun/lowercase_locale_ids

use lowercase locale ids except for default locale 'en-US'
bump 1.18.2