Fix issue with aborted rake task when packaging gem.

When attempting to package the gem, the following error is thrown and the task is aborted.
This patch updates the Rakefile to use the correct task for packaging.

$ rake package:gem
rake aborted!
ERROR: 'rake/gempackagetask' is obsolete and no longer supported. Use 'rubygems/package_task' instead.
/home/acline/Development/puppetlabs_spec_helper/Rakefile:3:in `<top (required)>'
/home/acline/.rvm/gems/ruby-1.9.3-p194/bin/ruby_noexec_wrapper:14:in `eval'
/home/acline/.rvm/gems/ruby-1.9.3-p194/bin/ruby_noexec_wrapper:14:in `<main>'
(See full trace by running task with --trace)