Remove beta from 9.3 image

.travis.yml

@@ -9,13 +9,13 @@ env:
   - FASTLANE_LANE=ci_commit
 matrix:
   include:
-    - osx_image: xcode9.3beta
+    - osx_image: xcode9.3
       env: FASTLANE_LANE=code_coverage FASTLANE_ENV=default
-    - osx_image: xcode9.3beta
+    - osx_image: xcode9.3
       env: FASTLANE_ENV=ios11_xcode9
-    - osx_image: xcode9.3beta
+    - osx_image: xcode9.3
       env: FASTLANE_ENV=tvos11_xcode9
-    - osx_image: xcode9.3beta
+    - osx_image: xcode9.3
       env: FASTLANE_ENV=osx
     - osx_image: xcode9.2
       env: FASTLANE_ENV=ios11_xcode9