bump version number to 0.4.0

homebrew is installing a 2 year old version of `rbenv-gemset` which is lacking awesome features like:

* bash completion for subcommands
* the use of `.ruby-version`
* the super handy `RBENV_GEMSET_FILE` env variable

In order to fix the `homebrew` formula, I need there to be a git tag pointing to how awesome this plugin is right now.

This pull request bumps the version number stored in `bin/rbenv-gemset`. I also added a tag to my fork, but am not 100% certain this will get dragged along with the pull request.

Thanks!