chore(package): update babel-plugin-istanbul to version 4.0.0

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -61,7 +61,7 @@
     "babel-core": "^6.21.0",
     "babel-eslint": "^7.1.1",
     "babel-plugin-flow-react-proptypes": "^0.21.0",
-    "babel-plugin-istanbul": "^3.1.2",
+    "babel-plugin-istanbul": "^4.0.0",
     "babel-plugin-transform-react-constant-elements": "^6.9.1",
     "babel-plugin-transform-runtime": "^6.15.0",
     "babel-preset-es2015": "^6.18.0",