Update CHANGELOG

Move Rails 4.1.9 changes to version 7.9.

CHANGELOG

 v 7.9.0 (unreleased)
   - Move labels/milestones tabs to sidebar
+  - Upgrade Rails gem to version 4.1.9.
   - Improve UI for commits, issues and merge request lists
   - Fix commit comments on first line of diff not rendering in Merge Request Discussion view.
 
@@ -59,7 +60,6 @@ v 7.8.0 (unreleased)
   - Show assignees in merge request index page (Kelvin Mutuma)
   - Link head panel titles to relevant root page.
   - Allow users that signed up via OAuth to set their password in order to use Git over HTTP(S).
-  - Upgrade Rails gem to version 4.1.9.
   - Show users button to share their newly created public or internal projects on twitter
   - Add quick help links to the GitLab pricing and feature comparison pages.
   - Fix duplicate authorized applications in user profile and incorrect application client count in admin area.