chore(package): update babel-plugin-istanbul to version 3.1.2

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -53,7 +53,7 @@
     "babel-cli": "^6.18.0",
     "babel-core": "^6.18.2",
     "babel-eslint": "^7.1.1",
-    "babel-plugin-istanbul": "^3.0.0",
+    "babel-plugin-istanbul": "^3.1.2",
     "babel-plugin-transform-runtime": "^6.15.0",
     "babel-preset-es2015": "^6.18.0",
     "babel-preset-flow": "^1.0.0",