Fix to recognise ‘ios’ as a device in the capabilities list

The documentation here https://github.com/appium/appium/blob/master/docs/caps.md states that ‘ios’ can be specified as a device. However, when using that device Appium fails with the error: “A new session could not be created. (Original error: A valid device type is required in the capabilities list) (Selenium::WebDriver::Error::WebDriverError)”.