Update to Swift 3.1.1; add 6/29 snapshot builds to Travis; Kitura/Kitura#1121

Author: quanvo87

.swift-version

@@ -1 +1 @@
-3.1
+3.1.1

.travis.yml

@@ -1,9 +1,3 @@
-# See the following URLs for further details on Travis CI
-# https://docs.travis-ci.com/user/customizing-the-build/
-# https://docs.travis-ci.com/user/docker/
-# https://docs.travis-ci.com/user/multi-os/
-
-# whitelist (branches that should be built)
 branches:
   only:
     - master
@@ -22,11 +16,14 @@ matrix:
     - os: linux
       dist: trusty
       sudo: required
-      env: SWIFT_SNAPSHOT=4.0-DEVELOPMENT-SNAPSHOT-2017-06-11-a
+      env: SWIFT_SNAPSHOT=4.0-DEVELOPMENT-SNAPSHOT-2017-06-29-a
     - os: osx
       osx_image: xcode8.3
       sudo: required
-      env: SWIFT_SNAPSHOT=4.0-DEVELOPMENT-SNAPSHOT-2017-06-11-a
+    - os: osx
+      osx_image: xcode9
+      sudo: required
+      env: SWIFT_SNAPSHOT=4.0-DEVELOPMENT-SNAPSHOT-2017-06-29-a
 
 before_install:
   - git clone https://github.com/IBM-Swift/Package-Builder.git