Update CHANGELOG.md for 9.1.3

[ci skip]

Former-commit-id: b09460db

CHANGELOG.md

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 ## 9.1.3 (2017-05-05)
+- Do not show private groups on subgroups page if user doesn't have access to.
 - Enforce project features when searching blobs and wikis.
 - Fixed branches dropdown rendering branch names as HTML.
 - Make Asciidoc & other markup go through pipeline to prevent XSS.