ci: switch from xcode9.3-moar to xcode8.3

Author: pietroalbini

.travis.yml

@@ -35,7 +35,6 @@ matrix:
         NO_DEBUG_ASSERTIONS=1
         CI_JOB_NAME=dist-x86_64-apple-alt
       os: osx
-      osx_image: xcode9.3-moar
       if: branch = auto
 
     # macOS builders. These are placed near the beginning because they are very
@@ -56,7 +55,6 @@ matrix:
         NO_DEBUG_ASSERTIONS=1
         CI_JOB_NAME=x86_64-apple
       os: osx
-      osx_image: xcode9.3-moar
       if: branch = auto
 
     - env: >
@@ -70,7 +68,6 @@ matrix:
         NO_DEBUG_ASSERTIONS=1
         CI_JOB_NAME=i686-apple
       os: osx
-      osx_image: xcode9.3-moar
       if: branch = auto
 
     # OSX builders producing releases. These do not run the full test suite and
@@ -90,7 +87,6 @@ matrix:
         NO_DEBUG_ASSERTIONS=1
         CI_JOB_NAME=dist-i686-apple
       os: osx
-      osx_image: xcode9.3-moar
       if: branch = auto
 
     - env: >
@@ -104,7 +100,6 @@ matrix:
         NO_DEBUG_ASSERTIONS=1
         CI_JOB_NAME=dist-x86_64-apple
       os: osx
-      osx_image: xcode9.3-moar
       if: branch = auto
 
     # Linux builders, remaining docker images