Add Capybara.server_port to spec_helper example

My browser based 302 redirects were failing with billy until I figured out that my VCR cassettes had the randomly chosen capybara server_port burned into them.  Once I set `Capybara.server_port` to a constant value, my subsequent tests played back from those cassettes worked just fine.

Context:  I discovered while playing with billy that I could disable billy's cache because VCR could now see all of my capybara-webkit browser requests.