Update travis image to Xcode 11.3 for Swift 5.1

Author: sunshinejr

.travis.yml

@@ -2,7 +2,7 @@ language: generic
 matrix:
   include: 
     - os: osx
-      osx_image: xcode11
+      osx_image: xcode11.3
       name: Danger
       env:
         - CACHE_NAME=SWIFT5_1
@@ -19,7 +19,7 @@ matrix:
         - brew install danger/tap/danger-swift
       script: DEBUG="*" danger-swift ci  
     - os: osx
-      osx_image: xcode11
+      osx_image: xcode11.3
       name: Swift 5.1 compatibility
       env:
         - CACHE_NAME=SWIFT5_1