Only require Slack's include files if they exists

[RMarks]

When on a local vvv environment, the following error is found:

Warning: require_once(/srv/www/wordpress-meta-environment/meta-repository/wordcamp.org/public_html/wp-content/mu-plugins/includes/slack/send.php): failed to open stream: No such file or directory in /srv/www/wordpress-meta-environment/meta-repository/wordcamp.org/public_html/wp-content/mu-plugins/0-slack.php on line 9

Plan is to only require if the file exists.

[RMarks]

Using is_readable() vs file_exists()

[coreymckrill]

In 7960:

WordCamp: Only require Slack's include file if it exists

This avoids an error in local environments where the Slack include file
may not exist.

Fixes #3987
Props RMarks