Update .travis-install.sh to Swift 4.1.1.

.travis-install.sh

@@ -32,7 +32,7 @@ if [ "$TRAVIS_OS_NAME" == "osx" ]; then
   PROTOC_URL=https://github.com/google/protobuf/releases/download/v3.5.1/protoc-3.5.1-osx-x86_64.zip
 else
   # Install swift
-  SWIFT_URL=https://swift.org/builds/swift-4.0.3-release/ubuntu1404/swift-4.0.3-RELEASE/swift-4.0.3-RELEASE-ubuntu14.04.tar.gz
+  SWIFT_URL=https://swift.org/builds/swift-4.1.1-release/ubuntu1404/swift-4.1.1-RELEASE/swift-4.1.1-RELEASE-ubuntu14.04.tar.gz
   echo $SWIFT_URL
   curl -fSsL $SWIFT_URL -o swift.tar.gz 
   tar -xzf swift.tar.gz --strip-components=2 --directory=local