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 issue #23 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: