Change updater script

[billerickson]

Now that the repo is public, we no longer need to host a zip and json file at sharecountplugin.com. @jaredatch saw a script that used GH tags, which would work well.

We'll use the 1.6.0 release to change the updater script.

[jaredatch]

https://github.com/FacetWP/github-updater-lite is the one I saw