Fail plugin activation if CMB2 is not found

[jazzsequence]

Currently, if you use Composer to install Games Collector, CMB2 is installed as a plugin. Because it's a required library of the plugin and since I don't bundle it in the zip file or anything, activation fatals the site because CMB2 is not active.

The workaround is to activate CMB2 first (or install as an mu-plugin), but the plugin should handle this.

Proposed solution

1. Deactivate the plugin if CMB2 is not active
2. Use the new plugin dependencies feature to require that CMB2 is active