Update CHANGELOG.md for 10.3.9

[ci skip]

CHANGELOG.md

@@ -492,6 +492,15 @@ entry.
 - Use a background migration for issues.closed_at.
 
 
+## 10.3.9 (2018-03-16)
+
+### Security (3 changes)
+
+- Fixed some SSRF vulnerabilities in services, hooks and integrations. !2337
+- Update nokogiri to 1.8.2. !16807
+- Fix GitLab Auth0 integration signing in the wrong user.
+
+
 ## 10.3.8 (2018-03-01)
 
 ### Security (1 change)