chore(package): update babel-plugin-istanbul to version 5.2.0

Closes #11

Author: greenkeeper[bot]

package.json

@@ -49,7 +49,7 @@
   },
   "devDependencies": {
     "babel-cli": "^6.26.0",
-    "babel-plugin-istanbul": "^4.1.5",
+    "babel-plugin-istanbul": "^5.2.0",
     "babel-preset-env": "^1.6.1",
     "coveralls": "^3.0.0",
     "cross-env": "^5.1.3",