fix path to 'required' file

Fix for the following error I was receiving on ruby 1.9.3 ...

    ../gems/activesupport-3.2.14/lib/active_support/dependencies.rb:251:in `require': cannot load such file -- user_agent (LoadError)
    from ../gems/activesupport-3.2.14/lib/active_support/dependencies.rb:251:in `block in require'
    from ../gems/activesupport-3.2.14/lib/active_support/dependencies.rb:236:in `load_dependency'
    from ../gems/activesupport-3.2.14/lib/active_support/dependencies.rb:251:in `require'
    from ../gems/capybara-user_agent-0.0.1/lib/capybara-user_agent.rb:2:in `<top (required)>'
    ... rest of stack trace ...