Add Swift 5 builds using SWIFT_DEVELOPMENT_SNAPSHOT (#16)

Author: djones6

.travis.yml

@@ -27,6 +27,11 @@ matrix:
       sudo: required
       services: docker
       env: DOCKER_IMAGE=ubuntu:16.04
+    - os: linux
+      dist: trusty
+      sudo: required
+      services: docker
+      env: DOCKER_IMAGE=ubuntu:16.04 SWIFT_SNAPSHOT=$SWIFT_DEVELOPMENT_SNAPSHOT
     - os: linux
       dist: trusty
       sudo: required
@@ -43,6 +48,10 @@ matrix:
     - os: osx
       osx_image: xcode10
       sudo: required
+    - os: osx
+      osx_image: xcode10.1
+      sudo: required
+      env: SWIFT_SNAPSHOT=$SWIFT_DEVELOPMENT_SNAPSHOT
 
 before_install:
   - git clone https://github.com/IBM-Swift/Package-Builder.git