Migrate to Xcode 9

Author: ikesyo

.swift-version

@@ -1 +1 @@
-3.1
+4.0

.travis.yml

@@ -5,20 +5,14 @@ branches:
     - master
 matrix:
   include:
-    - os: osx
-      osx_image: xcode8.3
-      xcode_scheme: ReactiveTask
-      script:
-        - set -o pipefail && xcodebuild -workspace "$TRAVIS_XCODE_WORKSPACE" -scheme "$TRAVIS_XCODE_SCHEME" test | xcpretty
-      env: JOB=Xcode8
     - os: osx
       osx_image: xcode9
       xcode_scheme: ReactiveTask
       script:
         - set -o pipefail && xcodebuild -workspace "$TRAVIS_XCODE_WORKSPACE" -scheme "$TRAVIS_XCODE_SCHEME" test | xcpretty
       env: JOB=Xcode9
     - os: osx
-      osx_image: xcode8.3
+      osx_image: xcode9
       script:
         - swift build
         - SWIFTPM_TEST_ReactiveTask=YES swift test
@@ -44,4 +38,4 @@ deploy:
   on:
     repo: Carthage/ReactiveTask
     tags: true
-    condition: $JOB = Xcode8
+    condition: $JOB = Xcode9