Use Xcode 9.2 for CI. Update CI config.

.travis.yml

@@ -1,12 +1,16 @@
 language: objective-c
-osx_image: xcode9
+osx_image: xcode9.2
 before_install: true
 install: true
 branches:
   only:
     - master
 script:
   - script/build
+cache:
+  directories:
+    - $HOME/Library/Caches/org.carthage.CarthageKit/dependencies
+    - Carthage/Build
 xcode_workspace: ReactiveObjC.xcworkspace
 matrix:
   include:
@@ -24,41 +28,19 @@ matrix:
       env:
         - XCODE_SDK=appletvsimulator
         - XCODE_ACTION="build-for-testing test-without-building"
-        - XCODE_DESTINATION="platform=tvOS Simulator,name=Apple TV 1080p"
+        - XCODE_DESTINATION="platform=tvOS Simulator,name=Apple TV"
     - xcode_scheme: ReactiveObjC-watchOS
       env:
         - XCODE_SDK=watchsimulator
         - XCODE_ACTION=build
-        - XCODE_DESTINATION="platform=watchOS Simulator,name=Apple Watch - 38mm"
-    - script:
-        - brew update
-        - brew outdated carthage || brew upgrade carthage
-        - carthage build --no-skip-current --platform mac
-      env:
-        - JOB=CARTHAGE-macOS
-    - script:
-        - brew update
-        - brew outdated carthage || brew upgrade carthage
-        - carthage build --no-skip-current --platform iOS
-      env:
-        - JOB=CARTHAGE-iOS
-    - script:
-        - brew update
-        - brew outdated carthage || brew upgrade carthage
-        - carthage build --no-skip-current --platform tvOS
-      env:
-        - JOB=CARTHAGE-tvOS
-    - script:
-        - brew update
-        - brew outdated carthage || brew upgrade carthage
-        - carthage build --no-skip-current --platform watchOS
-      env:
-        - JOB=CARTHAGE-watchOS
+        - XCODE_DESTINATION="platform=watchOS Simulator,name=Apple Watch Series 3 - 38mm"
     - script:
       - pod repo update --silent
       - pod lib lint ReactiveObjC.podspec
       env:
         - JOB=PODSPEC
+    - stage: carthage
+      script: carthage build --cache-builds --no-skip-current
 notifications:
   email: false
   slack: