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

Closes #68

Author: greenkeeper[bot]

package.json

@@ -69,7 +69,7 @@
     "babel-loader": "8.0.0",
     "babel-eslint": "8.2.6",
     "babel-plugin-add-module-exports": "0.3.3",
-    "babel-plugin-istanbul": "^4.1.6",
+    "babel-plugin-istanbul": "^5.1.4",
     "body-parser": "^1.18.2",
     "chai": "4.1.2",
     "coveralls": "^3.0.0",