Update Objc-Test for Local & TravisCI

[erisu]

Issue

Running objc-tests locally fails due to the current version of Xcode that is installed.

In most cases, developers may upgrade to the latest stable release of Xcode as they are released.

In my case, one of my macOS environments is using Xcode 9.4, the latest stable release, which supports iOS 11.4. This version of iOS does not support iPhone 5, which is the current target simulator for the objc-tests.

With the current test configuration, running locally causes test failures.
While Travis CI is still configured with 9.2, tests remain to pass.

Goal

• Upgrade Travis Environment to Xcode 9.3
  • Test Results: https://travis-ci.org/erisu/cordova-ios/builds/419492463
• Upgrade Objc-Tests Simulator Target Device to iPhone 8

This will allow tests to pass successfully on Travis CI and locally when using Xcode 9.3 or higher.

Side Notes:

Travis CI configured with Xcode 9.4 fails to open the iPhone simulator. Therefore I have fallen back the the previous version 9.3.

Xcode 9.4 Test Results: https://travis-ci.org/erisu/cordova-ios/builds/419480866
Error:

$ open -b com.apple.iphonesimulator
LSGetApplicationForInfo() failed with error -10814 while trying to determine the application with bundle identifier com.apple.iphonesimulator.
The command "open -b com.apple.iphonesimulator" exited with 1.