feat(stryker): Upgrade to Stryker v4 (#33)

* feat(stryker): Upgrade to Stryker v4

`@stryker-mutator` v4 API has no breaking change for this tool.

`stryker-diff-runner` is now compatible with `@stryker-mutator` from v3 to v4.

Closes #27

* fix(test): Remove useless mock

README.md

@@ -17,10 +17,10 @@ You will need the following things properly installed on your computer :
 
 ### Installation
 
-This module required `@stryker-mutator/core@3` and `@stryker-mutator/api@3` on your project to work. Install it and these dependencies like so :
+This module required `@stryker-mutator/core` and `@stryker-mutator/api` >=3 on your project to work. Install it and these dependencies like so :
 
 ```
-$ npm add @stryker-mutator/core@3 @stryker-mutator/api@3 stryker-diff-runner
+$ npm add @stryker-mutator/core @stryker-mutator/api stryker-diff-runner
 ```
 
 If you want to use this module with an older version of Stryker, you can install the v1 package :