chore(package): update cross-env to version 6.0.2

Closes #48

Author: greenkeeper[bot]

package.json

@@ -54,7 +54,7 @@
   },
   "devDependencies": {
     "babel-plugin-istanbul": "^4.1.5",
-    "cross-env": "^5.0.1",
+    "cross-env": "^6.0.2",
     "esdoc": "^1.0.3",
     "esdoc-ecmascript-proposal-plugin": "^1.0.0",
     "esdoc-importpath-plugin": "^1.0.1",