Update Travis script.

Avoid caching External directory and force dependencies build every time.
Travis use Xcode 11.4.

.travis.yml

@@ -1,7 +1,7 @@
 language: objective-c 
 
 # macOS and Xcode Version
-osx_image: xcode11.1
+osx_image: xcode11.4
 
 # Build dependencies
 # Travis has trouble with python3, which SPIRV-Tools requires,
@@ -9,11 +9,6 @@ osx_image: xcode11.1
 install:
   - ./fetchDependencies -v --skip-spirv-tools-build
 
-# Cache built deps
-cache:
-  directories:
-    - External
-
 script:
   - xcodebuild -project MoltenVKPackaging.xcodeproj -scheme "MoltenVK Package"
   - xcodebuild -workspace Demos/Demos.xcworkspace -scheme "Cube-macOS"