Update gem with current bundler defaults

* Version is now stored in parallel_runner/version.rb
* Updated gemspec
* Added LICENSE.txt-- it's just MIT, as listed on the current github README.md
* Updated parallel_runner_helper.rb to use new mechanize syntax,
  namely Mechanize::Page#/
* Also, turn off SSL verify, since it's currently raising an exception on
  twitter.com for some reason.
* We don't (yet) have an rdoc task, or a task to push to rubygems.org