Use GitHub Actions instead of Travis

[roger-tan]

In order to update to GitHub Actions.

• Migrate the codebase to Swift 5. This should be easy to migrate when Add queue support and keypath support to ObserverSet #21 is merged.
• Create ci.yml with the following code under. I tested locally so it should work when the project is migrated to Swift 5.

name: TABObserverSet CI

 on:
   workflow_dispatch: # This allows you to manually run the workflow from GitHub's web UI
   pull_request:
     branches: [ '*' ] # This means this workflow will run when any PR is created or any changes are pushed to a PR, on any branch

 jobs:
   test_xcode11_ios13:
     name: Run tests on Xcode 11 and iOS 13
     runs-on: macOS-latest

     steps:
       - name: Checkout
         uses: actions/checkout@v1
       - name: Set Xcode version to 11.6
         run: sudo xcode-select -switch /Applications/Xcode_11.6.app
       - name: Build for testing
         run:  xcodebuild  build-for-testing -workspace Example/TABObserverSet.xcworkspace -scheme TABObserverSet-Example -configuration Debug -sdk iphonesimulator13.6 -destination "OS=13.6,name=iPhone 11 Pro"
       - name: Test on iPhone 11 Pro
         run:  xcodebuild test-without-building -workspace Example/TABObserverSet.xcworkspace -scheme TABObserverSet-Example -configuration Debug -sdk iphonesimulator13.6 -destination "OS=13.6,name=iPhone 11 Pro" -enableCodeCoverage YES