Add Swift 4 env var to Travis

.travis.yml

@@ -1,7 +1,3 @@
-# Travis CI build file for Kitura-redis.
-# Kitura runs on OS X and Linux (Ubuntu v15.10).
-
-# whitelist (branches that should be built)
 branches:
   only:
     - master
@@ -20,14 +16,17 @@ matrix:
     - os: linux
       dist: trusty
       sudo: required
-      env: SWIFT_VERSION=3.0.2
+      env: SWIFT_SNAPSHOT=$SWIFT_4_DEV_SNAPSHOT
     - os: osx
       osx_image: xcode8.3
       sudo: required
+    - os: osx
+      osx_image: xcode9
+      sudo: required
+      env: SWIFT_SNAPSHOT=$SWIFT_4_DEV_SNAPSHOT
 
 before_install:
   - git clone https://github.com/IBM-Swift/Package-Builder.git
-  - test -n "$SWIFT_VERSION" && echo "$SWIFT_VERSION" > .swift-version || echo "SWIFT_VERSION not set, using $(cat .swift-version)"
 
 script:
   - ./Package-Builder/build-package.sh -projectDir $TRAVIS_BUILD_DIR