Allow to use different drivers in the same test suite

In my test suite I use different drivers, including appium_capybara.

I get this message for tests that use other drivers (selenium for example)
```
NoMethodError: undefined method `browser_initialized?' for #<Capybara::Selenium::Driver:0x007fdcf1773028>
appium_capybara/lib/appium_capybara/ext/session_ext.rb:5:in `reset!'
```

This pull request fix it
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