👷🏻‍♀️⬆️ Update travis image to xcode9.3

Author: phatblat

.travis.yml

@@ -7,17 +7,16 @@
 #
 ---
 os: osx
-# 9.3 beta is latest available as of 2018-04-08
-osx_image: xcode9.3beta
+osx_image: xcode9.3
 language: objective-c
 
 matrix:
   fast_finish: true
   include:
-    - osx_image: xcode9.3beta
+    - osx_image: xcode9.3
       env:
         - SCHEME="ObjectiveGit Mac"
-    - osx_image: xcode9.3beta
+    - osx_image: xcode9.3
       env:
         - SCHEME="ObjectiveGit iOS"