Update Swift 4 snapshot for Travis

Author: youming-lin

.travis.yml

@@ -22,14 +22,14 @@ matrix:
     - os: linux
       dist: trusty
       sudo: required
-      env: SWIFT_SNAPSHOT=4.0-DEVELOPMENT-SNAPSHOT-2017-08-15-a
+      env: SWIFT_SNAPSHOT=4.0-DEVELOPMENT-SNAPSHOT-2017-09-04-a
     - os: osx
       osx_image: xcode8.3
       sudo: required
     - os: osx
       osx_image: xcode9
       sudo: required
-      env: SWIFT_SNAPSHOT=4.0-DEVELOPMENT-SNAPSHOT-2017-08-15-a
+      env: SWIFT_SNAPSHOT=4.0-DEVELOPMENT-SNAPSHOT-2017-09-04-a
 
 before_install:
     - git clone https://github.com/IBM-Swift/Package-Builder.git