Fix usage with Bundler and cleanup version

This cleans up the version by just moving it to the gemspec like other Capistrano modules. It also fixes an issue where Bundler would fail if the Gemspec specified the module as a git repository. It would load the gemspec, requiring `lib/capistrano-foreman.rb`, which in turn required `capistrano/version` which would fail (because Capistrano was not yet installed).

Both `Capistrano::Foreman::VERSION` and `capistrano/version` don't appear to be used anywhere else. I've blanked out the `lib/capistrano-foreman.rb` like other modules so that it just exists for Bundler auto-require to work.

Also, I left the version at what was previously set. But it seems like this should be 1.0.1 or at most 1.1.0. Not sure why it was bumped to 3.0.0. :confused: