[Travis] Update Swift 4 snapshot to '2017-07-27-a'

Author: nvzqz

.travis.yml

@@ -29,7 +29,7 @@ matrix:
       sudo: required
       dist: trusty
       env:
-        - SWIFT_VERSION="4.0-DEVELOPMENT-SNAPSHOT-2017-07-24-a"
+        - SWIFT_VERSION="4.0-DEVELOPMENT-SNAPSHOT-2017-07-27-a"
 env:
   global:
     - LC_CTYPE=en_US.UTF-8