update readme for v4.2.1

README.md

@@ -524,6 +524,7 @@ But it's only tested until version `0.10.x` because the build dependencies are n
 ## Release History
 |Version|Date|Description|
 |:--:|:--:|:--|
+|4.2.1|2019-07-22|Upgrade lodash to version 4.17.15 to suppress messages about unrelated security vulnerability|
 |4.2.0|2018-02-01|Add options.promiseValueSize for promise value. Thanks to [Ryan Roemer](https://github.com/ryan-roemer) for the pull [#84]; Added option `deleteOnExpire`; Added DefinitelyTyped Typescript definitions. Thanks to [Ulf Seltmann](https://github.com/useltmann) for the pulls [#90] and [#92]; Thanks to [Daniel Jin](https://github.com/danieljin) for the readme fix in pull [#93];  Optimized test and ci configs.|
 |4.1.1|2016-12-21|fix internal check interval for node < 0.10.25, thats the default node for ubuntu 14.04. Thanks to [Jimmy Hwang](https://github.com/JimmyHwang) for the pull [#78](https://github.com/mpneuried/nodecache/pull/78); added more docker tests|
 |4.1.0|2016-09-23|Added tests for different key types; Added key validation (must be `string` or `number`); Fixed `.del` bug where trying to delete a `number` key resulted in no deletion at all.|