Needs Component in Version.php

[OptrixAU]

As of Moodle 3 (and enforced in 3.01+), you need to add the following line to your version.php...

$plugin->component = 'enrol_invitation';

Plugins apparently now need to declare their own unique ID, made up of the type of the plugin, an underscore and the name of the plugin.

After making this change, the plugin installs correctly. Users can apply this manually, but it would of course be nice to see this added to the source.