Changed CLI to utilize Launchy to open the graph

This should patch the error issue I raised a few days ago.  I added a new Spec for the CLI class, updated the code to make the test pass, and added the new dependency on Launchy into the gemspec file.

This is my first pull request, so I've likely made all manner of mistakes.  Please let me know if there's anything I need to correct.
Or like this:

~~~ruby
require 'test_helper'
require_relative '../lib/sandi_meter/cli'

describe SandiMeter::CLI do
  let(:cli) { SandiMeter::CLI }

  describe '#execute' do
    before do
      ARGV.clear
    end

    context 'with the graph flag passed in' do
      before do
        ARGV << '-g'
      end

      it 'opens the graph in a web browser' do
        cli.should_receive(:open_in_browser)
        expect { cli.execute }.to raise_error(SystemExit)
      end
    end
  end
end
~~~

1. Add a little bit of space between lines, it is better for reading I think.
2. Using `should` in RSpec will be deprecated (as far as I know). In short: `should` is added to every object which makes the whole system to be different from the system that will be in production. https://www.relishapp.com/rspec/rspec-expectations/docs/syntax-configuration