add ios 11 device

Author: Chakwen Hong

test/browsers.json

@@ -53,6 +53,15 @@
     "browser_version": "11.1",
     "real_mobile": null
   },
+  "bs-ios-11": {
+    "base": "BrowserStack",
+    "os": "ios",
+    "os_version": "11.0",
+    "browser": "iphone",
+    "device": "iPhone 8",
+    "browser_version": null,
+    "real_mobile": true
+  },
   "bs-android-8": {
     "base": "BrowserStack",
     "os": "android",