chore(package): update babel-plugin-istanbul to version 5.1.4

Closes #55

Author: greenkeeper[bot]

package.json

@@ -28,7 +28,7 @@
   "devDependencies": {
     "babel-cli": "^6.26.0",
     "babel-eslint": "^8.2.6",
-    "babel-plugin-istanbul": "^4.1.6",
+    "babel-plugin-istanbul": "^5.1.4",
     "babel-plugin-transform-class-properties": "^6.24.1",
     "babel-plugin-transform-object-rest-spread": "^6.26.0",
     "babel-preset-env": "^1.7.0",