Add rubygems release rake tasks

puppetlabs_spec_helper(feature/add_gemspec*)$ bundle exec rake -vT
rake build    # Build puppetlabs_spec_helper-0.4.1.gem into the pkg directory
rake install  # Build and install puppetlabs_spec_helper-0.4.1.gem into system gems
rake release  # Create tag v0.4.1 and build and push puppetlabs_spec_helper-0.4.1.gem to Rubygems
rake spec     # Run RSpec code examples