update swift build version

Author: tayloraswift

.travis.yml

@@ -2,9 +2,9 @@ language: generic
 sudo: required
 dist: trusty
 env:
-- SWIFT_BRANCH=swift-4.0-branch SWIFT_VERSION=swift-4.0-DEVELOPMENT-SNAPSHOT-2017-07-06-a
+- SWIFT_BRANCH=swift-4.0-branch SWIFT_VERSION=swift-4.0-DEVELOPMENT-SNAPSHOT-2017-07-28-a
 install:
 - mkdir swift
 - curl https://swift.org/builds/$SWIFT_BRANCH/ubuntu1404/$SWIFT_VERSION/$SWIFT_VERSION-ubuntu14.04.tar.gz -s | tar xz -C swift &> /dev/null
-script: 
+script:
 - env PATH=$(pwd)/swift/$SWIFT_VERSION-ubuntu14.04/usr/bin:$PATH swift test -c release