Better dependencies and add bundler compatibility

Hi,

While using your gem in a Gemfile, I had an error because rubpcap was missing, but the error was not really easy to understand.

I improved the dependencies in your gemspec so when using bundle, rubpcap gets installed as a dependency.

Also, I added a Gemfile and Gemfile.lock to be sure everyone that works on this gem uses the same version of rubpcap, rspec, etc.

Cheers