Travis: Include test for private doker registery

Author: dannys42

.travis.yml

@@ -38,6 +38,11 @@ matrix:
       sudo: required
       services: docker
       env: DOCKER_IMAGE=swift:5.1 SWIFT_SNAPSHOT=5.1
+    - os: linux
+      dist: xenial
+      sudo: required
+      services: docker
+      env: DOCKER_IMAGE=docker.kitura.net/kitura/swift-ci:5.3.3 SWIFT_SNAPSHOT=5.3.3
     - os: linux
       dist: xenial
       sudo: required