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.
I have closed this issue before you made your comment as I needed to think more about it.

I would say, that putting the version of the plugin in a lonely .rb file is the *standard* way of doing things. By standard, I should stay this is the default structure generated with 'Bundler' and used in a lot

Personally I find it handy to get the version of a specific gem with `HockeyGoal::VERSION` without any need to load and parse the gem specification. 
You can use this behavior to add guards on specific code :monkey:, we use ourself this kind of trick inside the twitter input plugin.

But, to make it work we need to be able to correctly namespace the version file and with the current module structure this is not possible. 

I am looking at how rails are dealing with versions.