re-add cross-env

Author: timche

package.json

@@ -48,6 +48,7 @@
     "@babel/preset-env": "^7.1.0",
     "babel-core": "^7.0.0-bridge.0",
     "babel-jest": "^23.6.0",
+    "cross-env": "^5.2.0",
     "es-check": "^4.0.0",
     "eslint": "^5.7.0",
     "eslint-config-jest-files": "^0.1.3",