chore(package): update babel-plugin-istanbul to version 5.1.3

Closes #42

Author: greenkeeper[bot]

package.json

@@ -24,7 +24,7 @@
   "devDependencies": {
     "babel-cli": "^6.26.0",
     "babel-core": "^6.26.3",
-    "babel-plugin-istanbul": "^4.1.6",
+    "babel-plugin-istanbul": "^5.1.3",
     "babel-preset-env": "^1.7.0",
     "babel-preset-import-export": "^1.0.2",
     "chai": "^4.1.2",