Symlink ruby.exe to ruby

Rails 4 introduced the `bin/` directory to generated projects with executables `rails`, `bundle`, `rake`, etc. The file generated on windows has a link to an incorrect `ruby.exe` in the shebang line. See #134.

This PR adds a symlink so that `ruby.exe` can be called in addition to `ruby.
Merge pull request #139 from heroku/schneems/rails4-windows-fix

Symlink ruby.exe to ruby