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

Closes #106

Author: greenkeeper[bot]

package.json

@@ -28,7 +28,7 @@
   "devDependencies": {
     "ava": "^0.25.0",
     "babel-core": "^6.24.0",
-    "babel-plugin-istanbul": "^4.0.0",
+    "babel-plugin-istanbul": "^5.1.4",
     "babel-polyfill": "^6.23.0",
     "babel-preset-es2015": "^6.16.0",
     "babel-preset-stage-0": "^6.16.0",