Update Travis CI build to include SPM, Cocoapods, and Carthage verifi…

…cation steps

.travis.yml

@@ -1,5 +1,20 @@
 language: objective-c
-osx_image: xcode10.1
-xcode_workspace: OktaAuthSdk.xcworkspace
-xcode_scheme: OktaAuthNative iOS
-xcode_destination: platform=iOS Simulator,OS=latest,name=iPhone 8
+osx_image: xcode11.5
+jobs:
+  include:
+    - stage: Unit tests iOS
+      name: iOS
+      script:
+      - xcodebuild -workspace OktaAuthSdk.xcworkspace -scheme "OktaAuthNative iOS" -destination "platform=iOS Simulator,OS=latest,name=iPhone 11" clean test
+    - stage: Swift Package Manager validation
+      name: MacOS
+      script:
+      - swift test
+    - stage: Cocoapods validation
+      name: iOS
+      script:
+      - pod lib lint --allow-warnings
+    - stage: Carthage validation
+      name: iOS
+      script:
+      - carthage build --no-skip-current