ci: Enable xcode11 build

Author: djones6

.travis.yml

@@ -51,11 +51,10 @@ matrix:
     - os: osx
       osx_image: xcode10.2
       sudo: required
-# Pending Travis Xcode 11 image
-#    - os: osx
-#      osx_image: xcode11
-#      sudo: required
-#      env: SWIFT_SNAPSHOT=$SWIFT_DEVELOPMENT_SNAPSHOT
+    - os: osx
+      osx_image: xcode11
+      sudo: required
+      env: SWIFT_SNAPSHOT=$SWIFT_DEVELOPMENT_SNAPSHOT
 
 before_install:
   - git clone https://github.com/IBM-Swift/Package-Builder.git