chore(package): update babel-plugin-istanbul to version 4.1.3

Closes #65

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -59,7 +59,7 @@
     "babel-core": "6.21.0",
     "babel-loader": "6.2.10",
     "babel-plugin-external-helpers": "6.18.0",
-    "babel-plugin-istanbul": "3.1.2",
+    "babel-plugin-istanbul": "4.1.3",
     "babel-plugin-transform-es2015-modules-umd": "6.18.0",
     "babel-preset-es2015": "6.18.0",
     "chai": "3.5.0",