Restrict dashboard.js to only enqueue on Dashboard (index.php)

This resolves an issue where dashboard.js was being included on every
page, and the Heartbeat Ajax response would include a PHP notice:

> Notice: Undefined index: wp-stream-heartbeat-last-id in .../stream/includes/admin.php on line 792
Merge pull request #408 from x-team/restrict-dashboard-js-enqueue

Restrict dashboard.js to only enqueue on Dashboard (index.php)