chore(package): update cross-env to version 5.1.1

Closes #12

Author: greenkeeper[bot]

package.json

@@ -13,7 +13,7 @@
     "ava": "^0.16.0",
     "babel-eslint": "^7.1.0",
     "codecov": "^1.0.1",
-    "cross-env": "^3.1.3",
+    "cross-env": "^5.1.1",
     "doctoc": "^1.2.0",
     "nyc": "^8.4.0",
     "standard": "^8.5.0",