update Travis image version to Xcode 9.3 final (ReSwift#338)

Author: DivineDominion

.travis.yml

@@ -18,12 +18,12 @@ matrix:
   include:
     - osx_image: xcode9
       env: SCHEME="macOS"  SDK="macosx10.13"          DESTINATION="arch=x86_64"                    SWIFT_VERSION="3.2"
-    - osx_image: xcode9.3beta
-      env: SCHEME="macOS"  SDK="macosx10.13"          DESTINATION="arch=x86_64"                    SWIFT_VERSION="4.0"
-    - osx_image: xcode9.3beta
-      env: SCHEME="iOS"    SDK="iphonesimulator11.3"  DESTINATION="OS=11.3,name=iPhone 8"          SWIFT_VERSION="4.0"
-    - osx_image: xcode9.3beta
-      env: SCHEME="tvOS"   SDK="appletvsimulator11.3"  DESTINATION="OS=11.3,name=Apple TV 4K"      SWIFT_VERSION="4.0"
+    - osx_image: xcode9.3
+      env: SCHEME="macOS"  SDK="macosx10.13"          DESTINATION="arch=x86_64"                    SWIFT_VERSION="4.1"
+    - osx_image: xcode9.3
+      env: SCHEME="iOS"    SDK="iphonesimulator11.3"  DESTINATION="OS=11.3,name=iPhone 8"          SWIFT_VERSION="4.1"
+    - osx_image: xcode9.3
+      env: SCHEME="tvOS"   SDK="appletvsimulator11.3"  DESTINATION="OS=11.3,name=Apple TV 4K"      SWIFT_VERSION="4.1"
 
 script:
   - set -o pipefail