Update ChangeLog

Author: sebastianbergmann

ChangeLog-5.7.md

@@ -7,6 +7,7 @@ All notable changes of the PHPUnit 5.7 release series are documented in this fil
 ### Fixed
 
 * Fixed [#1999](https://github.com/sebastianbergmann/phpunit/issues/1999): Handler is inherited from previous custom option with handler
+* Fixed [#2149](https://github.com/sebastianbergmann/phpunit/issues/2149): `assertCount()` does not handle generators properly
 * Fixed [#2478](https://github.com/sebastianbergmann/phpunit/issues/2478): Tests that take too long are not reported as risky test
 
 ## [5.7.14] - 2017-02-19