refresh Rakefile

- replace Gem::PackageTask with Bundler::GemHelper in
- add description to Rspec Rake task as well as add a common alias for test to simply run spec
refresh .gemspec

- ensure .gemspec is using latest Gem::Specification behaviour
- make sure that test gems are not a 'add_development_dependency'
use new Rake tasks
err on the safer side and use bundle exec instead of assuming --binstub'd installed gems
constrain to 1.8.7 compatible version of Rake

- http://docs.seattlerb.org/rake/History_rdoc.html#label-10.1.2+/+2014-03-24
Merge pull request #88 from karlfreeman/rake-gem-refresh

Refresh Rake gem and use new tasks

Conflicts:
	Gemfile
	twilio-ruby.gemspec