Use bundler gem tasks

```
$ rake -T | grep gem
rake build      # Build haml-4.1.0.beta.1.gem into the pkg directory
rake install    # Build and install haml-4.1.0.beta.1.gem into system gems
rake release    # Create tag v4.1.0.beta.1 and build and push haml-4.1.0.beta.1.gem to Rubygems
```