Update travis to xcode11.1 + iOS 13.1

Author: jkasten2

.travis.yml

@@ -1,6 +1,6 @@
 language: objective-c
-osx_image: xcode10
+osx_image: xcode11.1
 before_install:
   - cd iOS_SDK/OneSignalSDK
 script:
-  - xcodebuild -scheme UnitTests -sdk iphonesimulator -destination 'platform=iOS Simulator,name=iPhone 8 Plus,OS=12.0' test
+  - xcodebuild -scheme UnitTests -sdk iphonesimulator -destination 'platform=iOS Simulator,name=iPhone 8 Plus,OS=13.1' test