Add cross-env to package.json

Author: tristansnowsill

package.json

@@ -11,6 +11,7 @@
     "axios": "^0.16.2",
     "babel-plugin-transform-object-rest-spread": "^6.23.0",
     "bootstrap-sass": "^3.3.7",
+    "cross-env": "^5.0.1",
     "jquery": "^3.1.1",
     "laravel-mix": "^1.0",
     "lodash": "^4.17.4",