chore(ci): Don't use swiftenv on macOS

Author: kylef

.travis.yml

@@ -8,7 +8,7 @@ osx_image: xcode8.3
 env:
 - SWIFT_VERSION=3.1.1
 install:
-- eval "$(curl -sL https://gist.githubusercontent.com/kylef/5c0475ff02b7c7671d2a/raw/9f442512a46d7a2af7b850d65a7e9bd31edfb09b/swiftenv-install.sh)"
+- if [[ "$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
 script:
 - swift test