Replace util.isArray with Array.isArray

[troskoff]

util.isArray is deprecated since Node.js v4.0.0. Replace with stable Array.isArray.

[troskoff]

Guys, can I please get permission to contribute to the project? The fix is rather simple; I can create a PR with changes.

[tdumitrescu]

You'll want fork the repo and create a PR from your fork. Go to https://github.com/mixpanel/mixpanel-node and click on the "Fork" menu in the upper right corner of the UI. Thanks!

[troskoff]

Thank you for merging the changes. May I ask, do you plan to publish the new version of the library to NPM?