Update CHANGELOG.md for 8.15.7

[ci skip]

Former-commit-id: 62eee8f2

CHANGELOG.md

@@ -181,6 +181,10 @@ entry.
 - Add margin to markdown math blocks.
 - Add hover state to MR comment reply button.
+## 8.15.7 (2017-02-15)
+
+- No changes.
+
 ## 8.15.6 (2017-02-14)
 - Patch Asciidocs rendering to block XSS.