Add istanbul module into devDependencies (#66)

Author: kazuhitoyokoi

package.json

@@ -71,6 +71,7 @@
         "grunt-shell": "2.1.0",
         "grunt-simple-mocha": "0.4.1",
         "grunt-mocha-istanbul": "5.0.2",
+        "istanbul": "0.4.5",
         "should": "13.1.3",
         "node-red": "0.18.7",
         "node-red-node-test-helper": "0.1.8",