Fatal error when comment module is not installed

When the comment module is not installed and a stream is requested a PDOException occurs.

Steps to replicate problem:

1. Install Drupal with the minimal install profile.
2. drush dl ctools heartbeat-7.x-1.x
3. drush en heartbeat_defaults -y
4. navigate to /user/1/useractivity

PDOException: SQLSTATE[42S02]: Base table or view not found: 1146 Table 'drupal.comment' doesn't exist

This can be solved by checking if the comments module is enabled before joining the `comment` table in HeartbeatStream::createQuery().