updated CHANGELOG for 2.7.31

Author: fabpot

CHANGELOG-2.7.md

@@ -7,6 +7,10 @@ in 2.7 minor versions.
 To get the diff for a specific change, go to https://github.com/symfony/symfony/commit/XXX where XXX is the change hash
 To get the diff between two versions, go to https://github.com/symfony/symfony/compare/v2.7.0...v2.7.1
 
+* 2.7.31 (2017-07-05)
+
+ * bug #23378 [FrameworkBundle] Do not remove files from assets dir (1ed)
+
 * 2.7.30 (2017-07-03)
 
  * bug #23341 [DoctrineBridge][Security][Validator] do not validate empty values (xabbuh)