Bundler gem tasks

Instead of rolling own build & release rake tasks its better to use bundler's built-in tasks. 

Example `rake -T`:

```
rake build    # Build rails_best_practices-1.15.3.gem into the pkg directory
rake install  # Build and install rails_best_practices-1.15.3.gem into system gems
rake release  # Create tag v1.15.3 and build and push rails_best_practices-1.15.3.gem to Rubygems
```