add deviceName needed for appium

When using webdriverio for appium the DesiredCapability deviceName is 
needed.

types/webdriverio/index.d.ts

@@ -278,6 +278,7 @@ declare namespace WebdriverIO {
         platformVersion?: string;
         automationName?: string;
         app?: string;
+        deviceName?: string;
 
         cleanSession?: boolean;