Update cross-env requirement from ^5.2.0 to ^6.0.0 (#425)

Updates the requirements on [cross-env](https://github.com/kentcdodds/cross-env) to permit the latest version.
- [Release notes](https://github.com/kentcdodds/cross-env/releases)
- [Changelog](https://github.com/kentcdodds/cross-env/blob/master/CHANGELOG.md)
- [Commits](kentcdodds/cross-env@v5.2.0...v6.0.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

package.json

@@ -51,7 +51,7 @@
     "@types/react": "^16.8.19",
     "@types/react-dom": "^16.8.4",
     "@types/warning": "^3.0.0",
-    "cross-env": "^5.2.0",
+    "cross-env": "^6.0.0",
     "enzyme": "^3.3.0",
     "enzyme-to-json": "^3.4.0",
     "father": "^2.13.2",