If you are using capybara-webkit gem that you should install QT 4.7+, if you need to use pushState/replaceState. Also defining those functions with mock implementations is another good solution.
Guard with spork takes away a lot of the pain of using spork. Use the gem guard-spork which takes care of building you a sample guardfile. It takes care of restarting the appropriate framework you have in use be it test unit, rspec or cucumber.