Updated the CHANGELOG

CHANGELOG.md

+- Role: edxapp
+  - Removed deprecated variables EDXAPP_PLATFORM_TWITTER_URL, EDXAPP_PLATFORM_MEETUP_URL, EDXAPP_PLATFORM_LINKEDIN_URL, and EDXAPP_PLATFORM_GOOGLE_PLUS_URL in favor of EDXAPP_SOCIAL_MEDIA_FOOTER_URLS.  These variables haven't been used in edx-platform since March 17, 2015 (when https://github.com/edx/edx-platform/pull/7383 was merged).  This change is backwards incompatible with versions of edx-platform from before March 17, 2015.
+  - Added EDXAPP_MOBILE_STORE_URLS and EDXAPP_FOOTER_ORGANIZATION_IMAGE variables, used in https://github.com/edx/edx-platform/pull/8175 (v3 version of the edx.org footer).
+
 - Updated ansible fork with small bug fix.
   - https://github.com/ansible/ansible/pull/10957