Re-evaluate plugin re-activation method

Many users are reporting that plugins aren't being reactivated after being updated.

The likely culprit is our reactivation mechanism — [POSTing to ourself](https://github.com/humanmade/WP-Remote-WordPress-Plugin/blob/master/wprp.plugins.php#L108). Some security plugins restrict these requests, which means the plugin isn't reactivated.

We should reconsider this mechanism.