Move the version of the gem from the gemspec to a ruby file and drop support for the `milestone` concept.

Proposal for declaring the version of the gem inside a ruby file.
This will remove the obligation to use the `milestone` method in the plugin and will allow us to migrate to semver.
Our options seem to be:
* Put the version in the .gemspec
* Put the version in a lonely .rb file

Because we use rubygems to install plugins, we implicitly can query the version of a plugin if we do option 1. 

What's the benefits to putting a version file in all plugins?