Add hello_world.rb to gemspec

Added to resolve the following error. 

    1.9.3p392 :001 > require 'job_interview'
    LoadError: cannot load such file -- /Users/gregwoods/.rvm/gems/ruby-1.9.3-p392/gems/job_interview-0.1.4/lib/job_interview/hello_world
      from /Users/gregwoods/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
      from /Users/gregwoods/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
      from /Users/gregwoods/.rvm/gems/ruby-1.9.3-p392/gems/job_interview-0.1.4/lib/job_interview.rb:3:in `<top (required)>'
      from /Users/gregwoods/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `require'
      from /Users/gregwoods/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `rescue in require'
      from /Users/gregwoods/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:35:in `require'
      from (irb):1
      from /Users/gregwoods/.rvm/rubies/ruby-1.9.3-p392/bin/irb:16:in `<main>'