Add a Travis CI task for Xcode 9.3 beta.

.travis.yml

@@ -1,8 +1,5 @@
 language: objective-c
 osx_image: xcode9
-before_install:
-  - brew uninstall carthage
-  - HOMEBREW_NO_AUTO_UPDATE=1 brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/6ae4f69a652fb0ecb102b0c9216378679a4f1b92/Formula/carthage.rb # 0.22.0
 install: true
 branches:
   only:
@@ -39,6 +36,28 @@ jobs:
           XCODE_ACTION=build
           XCODE_DESTINATION="platform=watchOS Simulator,name=Apple Watch - 38mm,OS=2.2"
           script/build
+    - osx_image: xcode9.3beta
+      script:
+        - XCODE_SCHEME=FlexibleDiff
+          XCODE_SDK=macosx
+          XCODE_ACTION="build test"
+          XCODE_DESTINATION="arch=x86_64"
+          script/build
+        - XCODE_SCHEME=FlexibleDiff
+          XCODE_SDK=iphonesimulator
+          XCODE_ACTION="build-for-testing test-without-building"
+          XCODE_DESTINATION="platform=iOS Simulator,name=iPhone 6s"
+          script/build
+        - XCODE_SCHEME=FlexibleDiff
+          XCODE_SDK=appletvsimulator
+          XCODE_ACTION="build-for-testing test-without-building"
+          XCODE_DESTINATION="platform=tvOS Simulator,name=Apple TV 1080p"
+          script/build
+        - XCODE_SCHEME=FlexibleDiff
+          XCODE_SDK=watchsimulator
+          XCODE_ACTION=build
+          XCODE_DESTINATION="platform=watchOS Simulator,name=Apple Watch - 38mm,OS=2.2"
+          script/build
     - script:
         - pod repo update --silent
         - pod lib lint FlexibleDiff.podspec