chore: Update CI to Swift 4

The library no longer supports Swift 3 in this version.

Author: kylef

.travis.yml

@@ -4,9 +4,9 @@ os:
 language: generic
 sudo: required
 dist: trusty
-osx_image: xcode8.3
+osx_image: xcode9
 env:
-- SWIFT_VERSION=3.1.1
+- SWIFT_VERSION=4.0
 install:
 - if [[ "$TRAVIS_OS_NAME" != "osx" ]]; then eval "$(curl -sL https://gist.githubusercontent.com/kylef/5c0475ff02b7c7671d2a/raw/9f442512a46d7a2af7b850d65a7e9bd31edfb09b/swiftenv-install.sh)"; fi
 - git submodule update --init --recursive