chore(package): update babel-plugin-istanbul to version 5.1.1

Author: greenkeeper[bot]

package.json

@@ -20,7 +20,7 @@
   "license": "MIT",
   "devDependencies": {
     "babel-plugin-add-module-exports": "^0.2.1",
-    "babel-plugin-istanbul": "^4.1.3",
+    "babel-plugin-istanbul": "^5.1.1",
     "babel-plugin-transform-es2015-modules-umd": "^6.18.0",
     "babel-preset-es2015": "^6.18.0",
     "coveralls": "^3.0.1",