chore(package): update babel-plugin-istanbul to version 5.1.2

Closes #125

Author: greenkeeper[bot]

package.json

@@ -43,7 +43,7 @@
   "devDependencies": {
     "babel-cli": "^6.26.0",
     "babel-eslint": "^8.2.3",
-    "babel-plugin-istanbul": "^4.1.6",
+    "babel-plugin-istanbul": "^5.1.2",
     "babel-plugin-transform-runtime": "^6.23.0",
     "babel-preset-env": "^1.6.1",
     "babel-register": "^6.26.0",