Use selenium driver by default

Capybara expects the browser object to be a selenium driver, not
an appium driver.

```ruby
# start a Pry session and paste the following code
require 'appium_capybara'

Capybara.register_driver(:appium) do |app|
    opts = Appium.load_appium_txt file: File.join(Dir.pwd, 'appium.txt')
    Appium::Capybara::Driver.new app, opts
end

Capybara.default_driver = :appium

capy_driver = Capybara.current_session.driver
capy_driver.browser
```

Notice that selenium methods will fail.

```ruby
> capy_driver.go_back
NoMethodError: undefined method `navigate' for #<Appium::Driver:0x007fafda14d570
```

These methods exist on the selenium driver and the browser is set to the appium driver. The changes in this Pull Request fix the default so the upstream capybara commands work.

```ruby
> capy_driver.go_back
post /back
```

@sbonebrake Let me know what you think of these changes. I think the rest of the appium_capybara code will need to be updated. The benefit of this approach is all the [upstream methods](https://github.com/jnicklas/capybara/blob/da85ae722632b22895d505d405110538588a09a7/lib/capybara/selenium/driver.rb) such as `go_back` work properly.